Musnad Ahmad 2:215

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from 'Urwah, from al-Miswar ibn Makhramah and 'Abd al-Rahman ibn 'Abd al-Qari, that they both heard 'Umar saying: I passed by Hisham ibn Hakim ibn Hizam while he was reciting Surat al-Furqan during the lifetime of the Messenger of Allah (may Allah bless him and grant him peace). I listened to his recitation, and behold, he was reciting it in many letters (dialects/modes) which the Messenger of Allah (may Allah bless him and grant him peace) had not taught me. I almost assaulted him during the prayer, but I waited until he finished. When he finished, I grabbed him by his garment and said: "Who taught you this surah that you are reciting?" He said: "The Messenger of Allah (may Allah bless him and grant him peace) taught it to me." I said to him: "You have lied, for by Allah, the Prophet (may Allah bless him and grant him peace) is the one who taught me this surah that you are reciting." So I set off leading him to the Prophet (may Allah bless him and grant him peace), and I said: "O Messenger of Allah, I heard this man reciting Surat al-Furqan in letters that you did not teach me, and you are the one who taught me Surat al-Furqan!" The Prophet (may Allah bless him and grant him peace) said: "Release him, O 'Umar. Recite, O Hisham." He recited to him the recitation that I had heard from him. The Prophet (may Allah bless him and grant him peace) said: "Thus it was revealed." Then the Prophet (may Allah bless him and grant him peace) said: "Recite, O 'Umar." So I recited the recitation that the Messenger of Allah (may Allah bless him and grant him peace) had taught me. He said: "Thus it was revealed." Then the Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, the Qur'an was revealed in seven letters, so recite from it what is easy."