Musnad Ahmad 926:2

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from al-Hakam, from Mujahid, from Ibn Abi Layla, from Ubayy ibn Ka'b: That the Prophet, peace and blessings of Allah be upon him, was at the pond of Banu Ghifar. He said: Then Gabriel came to him and said: "Indeed, Allah commands you to have your nation recite the Quran upon one dialect." He said: "I ask Allah for His pardon and forgiveness, for my nation cannot endure that." Then he came to him a second time and said: "Indeed, Allah commands you to have your nation recite the Quran upon two dialects." He said: "I ask Allah for His pardon and forgiveness, for my nation cannot endure that." Then he came to him a third time and said: "Indeed, Allah commands you to have your nation recite the Quran upon three dialects." The Messenger of Allah, peace and blessings of Allah be upon him, said: "I ask Allah for His pardon and forgiveness, for my nation cannot endure that." Then he came a fourth time and said: "Indeed, Allah commands you to have your nation recite the Quran upon seven dialects, so whichever dialect they recite upon, they have acted correctly."