sorry br. can't help you with the hadith, but will say this that if there is a hadith that says that, and i do not doubt it for a famous Scholar says so, then yet for something to be fardh, it's authenticity needs to be taken into account; wajib is what is mandatory/compulsory [just like fardh is] on a person, but it leaves the possibility of a difference of opinion i THINK, maybe due to the not absolutely conclusive evidence, thus for such a hadith to make a ruling fardh, it may have to be of the totally undoubted level of authenticity which is mutawaitr; anything below that could render a ruling wajib the best approach to see if that Scholars ruling is valid [for himself] is to see wether he is qualified as a Mujtahid scholar; if he is not, then he should not be doing ijtihad in the first place; if he is, then he himself could adhere to his jitihad but no one esle, for all other schools and opinions of mujtahids have been prohibbited by the consensus except the four traditional schools