Sayyiduna Abdullah (bin Mas'ud) (may Allah be pleased with him) says: "By Allah, no surah of the Qur'an was revealed except that I know where it was revealed, and no verse of it was revealed except that I know concerning what it was revealed; and if I knew of anyone more knowledgeable than me about the Book of Allah, whom camels could reach, I would go to him."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8397