[bridsanaeds]

Salamu aleykum

The shafi'is have interpreted "wa innahu lafisq" (that would be impiety) as it is explained in another verse in the same sura:

أَو فِسقًا أُهِلَّ لِغَيۡرِ ٱللهِ بِهِ

The arabic for "and the meat of animals blasphemously dedicated to other than God." is "aw fisqan uhilla li ghayri 'llah"
This ayah thus explains the meaning of fisq in ayah 121.

another evidence used by the shafi'is is the hadith of A'isha radiya Allahu anha, that muslims were giving them meat, but they didn't know if they had mentioned the name of Allah over it when they slaughtered, and the Prophet salla Allahu aleyhi wa sallam said that they should mention the name of Allah over it and eat it. If tasmiyyah was a condition it would not be allowed to eat the meat since the asl for meat is hurma.

Assalamu Alaikum

Here is a response to both of the above explanations that attempt to make tasmiyyah sunnah rather than wajib (translated from Tafhimat Volume 3 by Maulana Maududi):

(Beginning of Tafhimat)

To establish their point of view, the Shafi'i scholars' first proof is from the verse from Surah Al-An'am, "Eat not of (meats) on which Allah’s name has not been pronounced: That would be fisq (impiety)."
وَلَا تَأۡڪُلُواْ مِمَّا لَمۡ يُذۡكَرِ ٱسۡمُ ٱللَّهِ عَلَيۡهِ وَإِنَّهُ لَفِسۡقٌ۬
The Shafi'i scholars argue that the letter waw—و—meaning "and" in و إنه لفسق cannot be taken as a conjunction (ataf) because the first portion of the verse (wa la ta'kulu mimma lam yuzkarism ullahi alayh—و لا تاكلوا مما لم يذكراسم الله عليه) is jumlah fi'liyyah insha'iyyah (an imperative sentence—i.e. a sentence in which a command is issued) and the latter portion (innahu la fisq—إنه لفسق) is a jumlah ismiyyah khabriyyah (nominal declarative—or simply put, a statement), and that it is incorrect to conjoin (i.e. join via conjunction) an insha'iyyah and an ismiyyah khabriyyah sentence according to the principles of elocution. With this reasoning, they consider the waw as a waw of hal (the conditional or subordinating conjunction which means "if"), and make the definition of the verse, "Do not eat of (meats), if it is fisq." The Shafi'is then define the word fisq with reference to verse 145 of Surah Al-An'am: "(Do not eat) what is fisq—(meat) on which a name other than Allah's has been invoked."
أَو فِسقًا أُهِلَّ لِغَيۡرِ ٱللهِ بِهِ
With such an explanation, they make the meaning of the verse such that only meat that has been slaughtered with a name other than Allah is haram; only the omission of taking Allah's name does not make it haram.
However, this is an extremely weak explanation against which numerous strong objections can be made. For one, if one simply reads this verse, his mind does not naturally come to the conclusion that the verse’s meaning is what is expounded by these scholars. Only if he approaches the verse with the mindset that the tasmiyyah is not obligatory will he be able to come to that deduction.
Secondly, if it is a violation of the Arabic elocutionary principles to join a jumlah ismiyyah khabriyyah and a jumlah fi'liyyah insha'iyyah with a conjunction, then according to what rules of elocution is the use of the word inna in innahu—إِنه (an emphatic word meaning "indeed") and a laam taakeed (the emphatic or intensifying laam in the word la fisq—لفسق) allowed in a jumlah haliyah (conditional sentence)? If Allah had wanted to say what the Shafi'is are saying, then he would have used the words wa huwa fisqوهو فسق (defined, "in the situation that it is fisq"), not wa innahu la fisq وإِنه لفسق (defined as "in the situation that it is indeed fisq")
Third, in their enthusiasm in to prove that tasmiyyah is merely sunnah, the Shafi'is, in stating that a jumlah ismiyyah khabriyyah and a jumlah fi'liyyah insha'iyyah cannot be conjoined, could not even keep in mind the full verse. The full verse in question is:
وَلَا تَأۡڪُلُواْ مِمَّا لَمۡ يُذۡكَرِ ٱسۡمُ ٱللَّهِ عَلَيۡهِ وَإِنَّهُ لَفِسقٌ۬ وَإِنَّ ٱلشَّيَـٰطِينَ لَيُوحُونَ إِلَىٰٓ أَوۡلِيَآٮِٕهِمۡ لِيُجَـٰدِلُوكُمۡ وَإِنۡ أَطَعۡتُمُوهُمۡ إِنَّكُمۡ لَمُشۡرِكُونَ
Eat not of (meats) on which Allah's name has not been pronounced: That would be fisq (impiety). The shayatin (evil devils) inspire their friends to contend with you, and if you were to obey them, you would indeed be mushrikun (Pagans).
Even if we take for granted that the waw in the verse is a waw of hal, it does not change the fact that a later waw in the verse (inو إِن اطعتموهم ) plays the part of an ataf (conjunction) conjoining a jumlah fi'liyyah insha'iyyah and a jumlah ismiyyah khabriyyah; the sentence afterwards (و إِن الشياطين ليوحون إِلى أوليآءهم—wa inna ashayatina layuhuna ila awliyaa'ihim; defined literally as "and the shayatin inspire their friends") is ismiyyah khabriyyah and cannot be made a haliyah (a conditional sentence) in any case, for such a sentence would make no sense; this becomes particularly problematic because the conjunction waw conjoins this ismiyyah khabriyyah to the jumlah fi'liyyah insha'iyyah that precedes it (و لا تاكلوا مما لم يذكراسم الله عليه). Moreover, this is not the only example of its kind in the Quran. At numerous places in the Quran, a jumlah fi'liyyah insha'iyyah is joined via conjunction to a jumlah ismiyyah khabriyyah. The following are only two examples of such an occurrence:
وَلَا تَنكِحُواْ ٱلۡمُشۡرِكَـٰتِ حَتَّىٰ يُؤۡمِنَّ وَلَأَمَةٌ۬ مُّؤمِنَةٌ خَيرٌ۬ مِّن مُّشرِكَةٍ۬ وَلَوۡ أَعۡجَبَتۡكُمۡ
And do not marry the idolatresses until they believe, and certainly a believing maid is better than an idolatress woman, even though she should please you.
The following verse is another one such example:
فَٱجلِدُوهُم ثَمَـٰنِينَ جَلدَةً۬ وَلَا تَقبَلُواْ لَهُمۡ شَہَـٰدَةً أَبَدً۬ا وَأُوْلَـٰٓٮِٕكَ هُمُ ٱلۡفَـٰسِقُونَ
Flog them by giving eighty stripes and do not admit any evidence from them ever; and these are the fasiqun (transgressors).
So the Shafi'is should either revise their idea of elocutionary principles or openly admit that the language of the Quran consistently violates their doctrine of elocution; it simply is not feasible to pronounce the waw in every sentence where the Quran joins a jumlah fi'liyyah insha'iyyah with a jumlah ismiyyah khabriyyah as a waw of hal and not an ataf, for many such verses would not make any sense.
Fourthly, if we accept the Shafi'is explanation of this verse, the full definition becomes, "Do not eat of (meats) on which Allah's name has not been pronounced, if indeed it is fisq—having had the name of someone other than Allah taken upon it." The question then becomes that if the original intent of this verse was to simply make only an animal that has been slaughtered with the name of other than Allah haram, then does not the first portion of the verse become completely useless, meaningless, and redundant? In such a situation, there is absolutely no reason to include the part of the verse that says "on which Allah's name has not been pronounced." Instead, the purpose of the verse can be fulfilled with just stating, "Do not eat of (meats) that have had the name of someone other than Allah taken on them." Can any intelligent person give a reasonable answer as to why there was any need to put مما لم يذكراسم الله عليه at the end of the verse?
Last, even if we accept the waw as haliyah, then there is no basis to accept the contention that the explanation of و إنه لفسق should come from the far-off verse اوفسقًا اهل لغير الله به. What prevents us from taking the designation of fisq in this verse as the one that would be normally found in the Arabic lexicon—that is disobedience and rebellion. In that case, the meaning of the verse "Do not eat of (meats) on which Allah's name has not been pronounced, if it is fisq" would be: "Do not eat of the meat in the case that one has intentionally avoided taking Allah's name on it," because fisq can only be applied to actions that are done with the intent to transgress and with the purpose to disobey, but it is not applied in the case of an accident. This interpretation is preferable to that of the Shafi'is for two reasons: it is consistent with all the verses and ahadith that have been revealed relevant to this issue, and because it saves the part of the verse مما لم يذكراسم الله عليه from becoming completely useless and excessive.
The second proof that the Shafi'i scholars provide is a narration by Aisha:
‏‏حدثنا ‏‏محمد بن عبيد الله ‏حدثنا ‏‏أسامة بن حفص المدني ‏عن هشام بن عروة عن أبيه عن عائشة ‏رضي الله عنها
‏أن قوما قالوا للنبي ‏‏صلى الله عليه وسلم إن قوما يأتونا باللحم لا ندري أذكر اسم الله عليه أم لا فقال ‏سموا عليه أنتم وكلوه ‏
‏قالت ‏وكانوا حديثي عهد بالكفر تابعه ‏علي ‏عن الدراوردي وتابعه ‏‏أبو خالد ‏‏والطفاوي
A group of people said to the Prophet, "some people bring us meat and we do not know whether they have mentioned Allah's name or not on slaughtering the animal." He said, "mention Allah's name on it and eat it." Those people had embraced Islam recently. (Sahih Bukhari, Sunan Abi Dawud, Sunan An-Nisa'i, Sunan Ibn Majah)
The Shafi'is deduce from this narration that tasmiyyah is not obligatory because if it were, the Prophet would have forbidden his Companions from eating it because of the doubtfulness that the name of Allah had been said upon it. However, this hadith in actuality disproves their own argument; it makes it clear that during the time of the Prophet, the Companions recognized the need to take the tasmiyyah on animals before slaughtering them—this is the reason that they came to the Prophet in the first place with this problem. If it were understood that the tasmiyyah was not necessary, why would this question even arise?—why would the Companions make an effort in asking a pointless question?
Even the Prophet's answer leads to the conclusion that tasmiyyah is compulsory. If not taking the name of Allah and taking the name of Allah were equal to each other and had no effect in making meat lawful, then the Prophet would have said this clearly in his answer, putting right his Companions' incorrect notion that tasmiyyah was a condition for making the meat halal. He would simply have said: "You can eat all types of meats, whether it has had the name of Allah said upon it or whether it has not had the name of Allah said upon it." Instead, the Prophet said, "mention Allah's name on it and eat it." The real meaning of the Prophet’s directive is that one should always consider a Muslim's meat as having been slaughtered in a correct way, and he should feel at ease when eating the Muslim's meat. But if he has some doubts as to the lawfulness of that meat, then he should allay his doubt—which is from the whisperings of Shaitan—by saying "Bismillah" before eating it.
It is evident that a Muslim cannot always determine the nature of another Muslim's meat, and neither does the Shariah require that he do so. It is impossible to establish solidly whether that meat has had the name of Allah said upon it, whether it has been slaughtered in a fashion complying with Islamic standards, or whether the slaughterer of the meat knows every single of the Islamic laws regarding animal slaughter. If one sets himself on this task, he will drive himself crazy.
A Muslim should always consider another Muslim's actions correct and without intentional malice. This holds true in all instances except when there is clear proof that the Muslim is not deserving of such unconditional acceptance. If some doubt arises without any substantial evidence, instead of acting on that doubt, the person should say "Bismillah" or "Astaghfirullah" and extinguish that misgiving. That is the message that this hadith offers. There is no evidence that stands up to the light that can prove that this narration indicates that tasmiyyah is not obligatory. (End of Tafhimat)

There are many other arguments that the hadith of Aisha does not mean that tasmiyyah is not wajib:

1. The people in the hadith were Muslims, and with a Muslim one generally assumes that he is doing things correctly. (I'la al-Sunan) For example, we will pray behind an imam, assuming he has wudu', even if we did not actually see him perform wudu', since we assume the best of him and have no grounds for suspicion. (Hafiz Ibn Hajar al Asqalani concurs with this view. He adds: “This is what is understood by the context of the hadith since the answer of the Prophet to the question was, ‘Say Bismillah and eat.’ It is as though they were told, ‘that is not your concern, rather what should concern you is to consume it (wholesomely in the sunnah manner) by saying Bismillah before partaking thereof.’” Hafiz Ibn Hajar adds, “Similarly, the slaughter of the Bedouin Muslims will be permissible (for consumption) since they usually know of the tasmiyyah (at the time of slaughter). Ibn Abd al-Barr has concluded, ‘In this hadith, it is understood that the slaughter of a Muslim should be consumed and he should be regarded as having taken tasmiyyah upon its slaughter (even when one is not certain about this fact) because with regards to a Muslim, one should entertain nothing but good thoughts unless concrete evidence is established to the contrary.’” (Fath al-Bari 9:793) Ibn Hajar goes on to show the variant readings of the hadith of Aisha in different compilations in which the Prophet actually explicitly explained the reason for saying “Bismillah” is actually because one should expect the best out of fellow Muslims: “(1) The narration of Ibn Uyayna has the addition, ‘accept their oaths and eat’; that clearly means that one should take their word for it that they have taken tasmiyyah upon slaughter (and partake without doubts). (2) The narration of Abu Sa’id: Imam Tabarani has recorded a narration of Abu Sa’id though with a difference in wording in which he says, ‘accept their word that they have effected slaughter.’ Explaining this hadith, Allamah ibn Tin comments: ‘Concerning tasmiyyah upon slaughter carried out by others of which they are unaware, there is no obligation upon them to verify its lawfulness. The slaughter will only be held incorrect when such evidence is established.’ Ibn Hajar adds, “Allah has not made it obligatory upon any Muslim to be aware of tasmiyyah upon the slaughter of another Muslim, since the slaughter of another Muslim will be always regarded as correct unless evidence is established to the contrary.” (Fath al-Bari 9:794)

2. It could be postulated—as has been done by some scholars—that the hadith is an evidence that mentioning the name of Allah is waived from someone who is unaware of the obligation (due to being new to Islam), just as it is waived from the forgetful one. (Al-Banayah Sharh Al-Hidayah)

3. Imam Malik has a different interpretation. He states, after narrating this hadith in his Muwatta', "That was in the beginning of Islam." I.e. he considered that the concession was later cancelled.

Wallahu A'lam