Musannaf Ibn Abi Shayba 22:209

Muhammad ibn Bishr narrated to us, he said: Isma'il ibn Abi Khalid narrated to us, he said: 'Abdullah ibn 'Isa narrated to me, from 'Abd al-Rahman ibn Abi Layla, he said: Ubayy ibn Ka'b informed me that the Messenger of Allah, may Allah bless him and grant him peace, was visited by Gabriel, who said: "Indeed, Allah commands you to have your nation recite the Qur'an according to seven modes (ahruf). So whichever mode they recite upon, they have hit the mark."