Musnad Ahmad 923:4

Abdullah narrated to us, Abu Bakr ibn Abi Shaybah narrated to us, Ubaydullah ibn Musa narrated to us, from Isra'il, from Abu Ishaq, from Suqayr al-Abdi, from Sulayman ibn Surad, from Ubayy ibn Ka'b, who said: I heard a man reciting, so I said: "Who taught you to recite?" He said: "The Messenger of Allah, may Allah bless him and grant him peace." I said: "Come with me to him." So I came to the Prophet, may Allah bless him and grant him peace, and said: "Ask him to recite." He said: "Recite." So he recited, and he said: "You have done well." I said to him: "Did you not teach me such and such?" He said: "Yes, and you have also done well." I gestured with my hand: "You have done well," twice. He said: The Prophet, may Allah bless him and grant him peace, struck my chest with his hand, then said: "O Allah, remove doubt from Ubayy." I broke out in a sweat, and my insides were filled with fear. The Messenger of Allah, may Allah bless him and grant him peace, said: "O Ubayy, two angels came to me. One of them said: 'Recite in one dialect.' The other said: 'Increase him.' I said: 'Increase me.' He said: 'Recite in two dialects.' The other said: 'Increase him.' I said: 'Increase me.' He said: 'Recite in three.' The other said: 'Increase him.' I said: 'Increase me.' He said: 'Recite in four dialects.' The other said: 'Increase him.' I said: 'Increase me.' He said: 'Recite in five dialects.' The other said: 'Increase him.' I said: 'Increase me.' He said: 'Recite in six.' The other said: 'Increase him.' He said: 'Recite in seven dialects.' For the Quran was revealed in seven dialects."