Musnad Abu Ya'la al-Mawsili 152:81

From al-Mughirah, from Wasil ibn Hayyan, from Abdullah ibn Abi al-Hudhayl, from Abu al-Ahwas, from Abdullah, from the Prophet (may Allah bless him and grant him peace): "If I were to take a close friend (khalil) from the people of the earth, I would have taken Abu Bakr ibn Abi Quhafah as a close friend, but your companion is the close friend (khalil) of Allah." And the Quran was revealed in seven ahruf (modes/dialects). For every verse of it, there is an outward meaning (zahr) and an inward meaning (batn), and for every limit (hadd), there is a point of emergence (matla').