From Sahih Bukhari (Hadith 5002):
Narrated `Abdullah (bin Mas`ud):By Allah other than Whom none has the right to be worshipped! There is no Sura revealed in Allah's Book but I know at what place it was revealed; and there is no Verse revealed in Allah's Book but I know about whom it was revealed. And if I know that there is somebody who knows Allah's Book better than I, and he is at a place that camels can reach, I would go to him
Reference: Bukhari 5002