Sahih Muslim Book 31, Hadith Number 6023.

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

Abdullah reported: By Him besides Whom there is no god, there is no chapter in the Book of Allah about which I do not know as to where it was revealed and there is no verse about which I do not know in what context it was revealed, and if I were to know of one having a better understanding of the Book of Allah than I (and I could reach him) on the back of the mule, I would have definitely gone to him on camel’s back.