Musnad Ahmad 2:196

'Abd al-Rahman narrated to us, from Malik, from al-Zuhri, from 'Urwah, from 'Abd al-Rahman ibn 'Abd, from 'Umar ibn al-Khattab, who said: I heard Hisham ibn Hakim reciting Surat al-Furqan in prayer in a way other than how I recite it, and the Messenger of Allah (peace be upon him) had taught it to me. I grabbed his garment and took him to the Messenger of Allah (peace be upon him) and said: "O Messenger of Allah, I heard him reciting Surat al-Furqan in a way other than how you taught it to me." He said: "Recite." So he recited the recitation that I had heard from him. He said: "Thus it was revealed." Then he said to me: "Recite." So I recited, and he said: "Thus it was revealed. Indeed, this Quran was revealed upon seven ahruf (modes/dialects), so recite what is easy of it."