Musnad Ahmad 551:21

Abu Salamah al-Khuza'i narrated to us, he said: 'Abdullah ibn Ja'far ibn 'Abd al-Rahman ibn al-Miswar ibn Makhramah informed us, he said: Yazid ibn 'Abdullah ibn Usamah ibn al-Had informed me, from Busr ibn Sa'id, from Abu Qays, the freed slave of 'Amr ibn al-'As, who said: 'Amr ibn al-'As heard a man reciting a verse from the Qur'an, so he said: "Who taught you to recite it?" He said: "The Messenger of Allah, may Allah bless him and grant him peace." He said: "The Messenger of Allah, may Allah bless him and grant him peace, taught me to recite it differently than this." So they both went to the Messenger of Allah, may Allah bless him and grant him peace. One of them said: "O Messenger of Allah, such and such a verse," and he recited it. The Messenger of Allah, may Allah bless him and grant him peace, said: "It was revealed thus." The other said: "O Messenger of Allah," and he recited it to the Messenger of Allah, may Allah bless him and grant him peace, and said: "Is it not thus, O Messenger of Allah?" He said: "It was revealed thus." The Messenger of Allah, may Allah bless him and grant him peace, then said: "Indeed, this Qur'an was revealed in seven modes (ahruf), so whichever of them you recite, you have acted correctly. Do not dispute regarding it, for disputing regarding it is disbelief," or "a sign of disbelief."