Musnad Ahmad 2:216

Al-Hakam ibn Nafi' narrated to us, Shu'ayb informed us, from al-Zuhri, 'Urwah narrated to me, regarding the hadith of al-Miswar ibn Makhramah and 'Abd al-Rahman ibn 'Abd al-Qari, that they both heard 'Umar ibn al-Khattab saying: I heard Hisham ibn Hakim ibn Hizam reciting Surat al-Furqan during the lifetime of the Prophet, 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 was about to assault him during the prayer, but I waited until he finished the prayer. When he finished... and he mentioned the rest of its meaning.