Musnad Ahmad 30:871

Anas ibn Iyad narrated to us, Abu Hazim narrated to me, from Abu Salamah—I do not know it except from Abu Hurairah—that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The Quran was revealed in seven ahruf (modes/dialects). Disputation regarding the Quran is disbelief"—three times. "So, whatever you recognize of it, act upon it; and whatever you are ignorant of, refer it to the one who knows it."