Musnad Abu Ya'la al-Mawsili 155:176

Zuhayr narrated to us, Anas bin 'Iyad narrated to us, from Abu Hazim, from Abu Salamah, who said: I do not know it except from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The Qur'an was revealed in seven ahruf (modes/dialects). And arguing about the Qur'an is disbelief," [he said it] three times. "Whatever you know of it, then act upon it; and whatever you are ignorant of it, then refer it to its scholar."