Musnad Ahmad 551:19

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, saying: ‘Abdullah bin Ja’far—meaning al-Makhrami—narrated to us, saying: Yazid bin ‘Abdullah bin Usamah bin al-Had narrated to us, from Busr bin Sa’id, from Abu Qays, the freed slave of ‘Amr bin al-‘As, from ‘Amr bin al-‘As, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The Qur’an was revealed in seven ahruf (modes/dialects). Whichever harf you recite in, you have acted correctly, so do not dispute regarding it, for disputing regarding it is disbelief."