How it is Known

The well-known asbab al-nuzul have been related to us by the reliable Companions of the Prophet Muhammad. Only reports which are sahih can be considered fully reliable, as is the case in the science of hadith generally. A particular condition here is also that the person who relates it should have been present at the time and occasion of the event (the revelation). [Wahidi. p.4.] Reports from tab'iun only, not going back to the Prophet and his Companions are to be considered weak (da'if). Hence one cannot accept the mere opinion of writers or people that such and such verse might have been revealed on such and such occasion. Rather one needs to know exactly who related this incident, whether he himself was present, and who transmitted it to us.