Musnad Ahmad 5:269

'Abdullah narrated to us, Abu Muhammad Sa'id ibn Muhammad al-Jarmi narrated to us—he came to us from Kufa—Yahya ibn Sa'id al-Umawi narrated to us, from al-A'mash, from 'Asim, from Zirr ibn Hubaysh. And Sa'id ibn Yahya ibn Sa'id narrated to me, my father narrated to us, al-A'mash narrated to us, from 'Asim, from Zirr ibn Hubaysh, who said: 'Abdullah ibn Mas'ud said: We disputed regarding a surah of the Qur'an, and we said: "Thirty-five verses, thirty-six verses." He said: So we went to the Messenger of Allah, peace and blessings of Allah be upon him, and we found 'Ali speaking privately with him. We said: "We have differed regarding the recitation." The face of the Messenger of Allah, peace and blessings of Allah be upon him, turned red, and 'Ali said: "Indeed, the Messenger of Allah, peace and blessings of Allah be upon him, commands you to recite as you were taught."