In order for someone to atttribute a saying to Nabi they are required to produce the chain for it. Until then, their attribution is worthless. Those opposing them aren't required to prove it to be fabricated, rather those quoting it have to prove its authenticity. To understand, if someone claimed that Nabi mentioned forums to be haram! If we ask him for the proof, he could claim that it is mentioned in hadith, even though he doesn't have a chain for it and that if you oppose him, please show which book of fabricated ahadith classified it as a fabrication! What would be a suitable reply to such an ignoramous?