There seems to be a commonly held belief that the prophet Muhammad (and the prophets in general) are somehow protected from ever committing any sin. I have seen nothing in the Qur'an to suggest this, nor am I aware of any authentic hadith to support this claim.
It has always been my understanding that the prophets are men like any other, specially chosen by the Almighty to relay His message but not necessarily granted any rights and privileges other than what He has explicitly stated; the Christians for example make a big deal about Jesus being sinless, but they have evidence from their own Scriptures to support this. Even if the other prophets were among the best men of their generation, they would still be susceptible to the same trials and temptations of any other man, and thus to sinning.
Note that I am not asking whether the prophets were forgiven for their sins; God is Al-Raheem so I have no difficulty accepting that. The question is about whether the prophets in general, or Muhammad (PBUH) in particular, ever sinned in the first place.
Where does this Islamic belief of sinlessness come from? Does it have an authentic source?