Abu al-Ahwas (may Allah have mercy on him) narrates: I came to Abu Musa al-Ash'ari (may Allah be pleased with him), and with him were Abdullah bin Mas'ud and Abu Mas'ud al-Ansari (may Allah be pleased with them both), and they were looking into a copy of the Qur'an. We talked for a while, then Abdullah (may Allah be pleased with him) went out, and Abu Mas'ud (may Allah be pleased with him) said: "No, by Allah, I do not know that the Messenger of Allah (peace and blessings be upon him) left (behind him) anyone more knowledgeable of the Book of Allah than this man who has just got up and left."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8464