The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fada'il Al-Sahabah)
Muslim :: Book 31 : Hadith 6020
Abu Ahwas reported: We were in the house of Abu Musa along with some of the companions of 'Abdullah and they were looking at the Holy Book. 'Abdullah stood up, whereupon Abu Mas'ud said: I do not know whether Allah's Messenger, (may peace be upon him) has left after him one having a better knowledge (of Islam) than the man who is standing. Abu Musa said: If you say this, that is correct, because he had been present when we had been absent and he was permitted when we were detained.