Musnad Ahmad 926:6

'Abdullah narrated to us, Abu Bakr ibn Abi Shaybah narrated to me, Ghundar narrated to us, from Shu'bah, from al-Hakam, from Mujahid, from 'Abd al-Rahman ibn Abi Layla, from Ubayy ibn Ka'b: "That Gabriel came to the Prophet, peace and blessings of Allah be upon him, and said: 'Indeed, Allah commands you to have your nation recite the Qur'an according to seven ahruf (modes/dialects). So whichever harf they recite upon, they have acted correctly.'"