Abu al-Ahwas (may Allah have mercy on him) narrates: We were in the house of Abu Musa (may Allah be pleased with him) with some of the Companions of the Messenger of Allah (peace and blessings be upon him), and they were looking into a copy of the Qur'an (mushaf). Meanwhile Abdullah (may Allah be pleased with him) stood up, and Abu Mas'ud (may Allah be pleased with him) said: "I do not know that the Messenger of Allah (peace and blessings be upon him) left behind him anyone more knowledgeable of what Allah, the Mighty and Majestic, revealed (the Qur'an) than this man who is standing (here)." At this Abu Musa (may Allah be pleased with him) said: "Since you have said this, (then know that) he used to be present when we were absent, and he used to be given permission when we were kept behind the curtain."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8462