Musnad Abu Ya'la al-Mawsili 152:335

Sahl ibn Zanjalah al-Razi narrated to us, Ibn Abi Uways narrated to us, from his brother, from Sulayman ibn Bilal, from Abu al-Ahwas, from Abdullah, from the Prophet (peace and blessings of Allah be upon him), who said: "The Quran was revealed upon seven ahruf (modes/dialects), for every harf of them there is an outward (zahr) and an inward (batn)."