Musnad Ahmad 26:772

Rawh narrated to us, Hammad ibn Salamah narrated to us, from ‘Asim ibn Bahdalah, from Zirr ibn Hubaysh, 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 of it. I said, "Who taught you to recite?" He said, "The Messenger of Allah (ﷺ) taught me to recite." I said, "The Messenger of Allah (ﷺ) taught me to recite such and such." So I went to the Messenger of Allah (ﷺ), and there was a man with him. I said, "O Messenger of Allah, did you not teach me to recite such and such?" He said, "Yes." The other man said, "Did you not teach me to recite such and such?" He said, "Yes." The face of the Messenger of Allah (ﷺ) changed color. The man who was with him said, "Let each one of you recite as he heard, for those who were before you were destroyed, or perished, only because of disagreement." I do not know if he commanded him to say that, or if it was something he said on his own.