Sahih Muslim Book 31, Hadith Number 6020.

Chapter : The merits of ‘Abdullah b. Mas’ud and that of his mother (Allah be pleased with them).

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.