Musnad Ahmad 2:77

'Abd al-A'la ibn 'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from 'Urwah ibn al-Zubayr, from al-Miswar ibn Makhramah, that 'Umar ibn al-Khattab said: I heard Hisham ibn Hakim ibn Hizam reciting Surat al-Furqan, and he recited letters in it that the Prophet of Allah had not taught me. I wanted to assault him while I was in prayer, but when he finished, I said: "Who taught you this recitation?" He said: "The Messenger of Allah (peace and blessings of Allah be upon him)." I said: "You have lied, by Allah, the Messenger of Allah (peace and blessings of Allah be upon him) did not teach you this recitation." So I took him by the hand, leading him, and I went with him to the Messenger of Allah (peace and blessings of Allah be upon him). I said: "O Messenger of Allah, you taught me Surat al-Furqan, and I heard this man reciting letters in it that you did not teach me." The Messenger of Allah (peace and blessings of Allah be upon him) said: "Recite, O Hisham." So he recited as he had recited. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Thus it was revealed." Then he said: "Recite, O 'Umar." So I recited, and he said: "Thus it was revealed." Then the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the Quran was revealed in seven ahruf (modes/dialects)."