Musnad Ahmad 914:2

'Affan narrated to us, he said: Hammad narrated to us, he said: Humayd informed us, from Anas, from 'Ubadah ibn al-Samit, that Ubayy ibn Ka'b said: The Messenger of Allah (may Allah bless him and grant him peace) taught me a verse, and he taught it to another person differently from Ubayy's recitation. I said: "Who taught it to you?" He said: "The Messenger of Allah (may Allah bless him and grant him peace) taught it to me." I said: "By Allah, he taught it to me such and such." Ubayy said: Nothing had ever stirred in my heart regarding Islam like what stirred that day. So I came to the Prophet (may Allah bless him and grant him peace) and said: "O Messenger of Allah, did you not teach me such and such a verse?" He said: "Yes." I said: "But this one claims that you taught him such and such." He struck my chest with his hand, and that [feeling] departed, and I did not find anything of it afterward. Then the Messenger of Allah (may Allah bless him and grant him peace) said: "Gabriel and Michael came to me. Gabriel said: 'Recite the Quran in one dialect.' Michael said: 'Ask him for more.' He said: 'Recite it in two dialects.' He said: 'Ask him for more,' until it reached seven dialects. He said: 'Each of them is healing and sufficient.'"