Musannaf Ibn Abi Shayba 27:105

Muhammad ibn Bishr narrated to us, he said: Isma'il ibn Abi Khalid narrated to us, he said: 'Abdullah ibn 'Isa narrated to us, from 'Abd al-Rahman ibn Abi Layla, he said: Ubayy ibn Ka'b informed me that the Prophet, may Allah bless him and grant him peace, said to him: "O Ubayy, indeed my Lord sent to me that I should recite the Qur'an in one dialect. So I replied to Him, 'Make it easy for my nation.' Then He replied to me, 'Recite the Qur'an in seven dialects, and for every reply I have replied to you, you have a request you may ask of Me.'" He said: I said, "O Allah, forgive my nation. O Allah, forgive my nation." And I delayed the third one for the day when the creation will yearn for me, even Ibrahim.