Musnad Ahmad 921:1

Yahya ibn Sa'id narrated to us, from Humayd, from Anas, from Ubayy ibn Ka'b, who said: Nothing has scratched my chest [caused me doubt] since I accepted Islam, except that I recited a verse, and a man recited it differently than my recitation. So we came to the Prophet, peace and blessings of Allah be upon him. I said: "Did you not teach me such-and-such a verse?" He said: "Yes." The other man said: "Did you not teach me such-and-such a verse?" He said: "Yes. Gabriel came to me, and Michael. Gabriel sat on my right, and Michael on my left. Gabriel said: 'Recite the Quran in one mode.' Michael said: 'Ask him for more,' until it reached seven modes, all of them sufficient and healing."