From Sahih Muslim (Hadith 6333):
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
Reference: Muslim 6333