Musnad Ahmad 26:442

Abd al-Samad and Affan narrated to us—the meaning is the same—they said: Hammad narrated to us. Affan said: Asim informed us, from Zirr, from Ibn Mas’ud, who said: The Messenger of Allah (ﷺ) taught me to recite Surah al-Ahqaf, and he taught another man to recite it. He differed with me in a verse, so I said to him: "Who taught you to recite it?" He said: "The Messenger of Allah (ﷺ)." So I came to him while he was among a group of people, and I said: "O Messenger of Allah, did you not teach me to recite such-and-such a verse?" He said: "Yes." I said: "But this man claims that you taught him to recite it as such-and-such?" The face of the Messenger of Allah (ﷺ) changed. The man who was with him said: "Let each man among you recite as he heard, for those who were before you were only destroyed by disagreement." He [Ibn Mas’ud] said: "By Allah, I do not know whether the Messenger of Allah (ﷺ) commanded him to say that, or if he said it himself."