Musnad Ahmad 3967
Musnad of Abdullah bin Mas'ood
It was narrated that Ibn Mas'ood said: The Messenger of Allah (ﷺ) taught me Soorat al-Ahqaf, and he taught it to another man, and he differed from me in his recitation of one verse. I said to him: Who taught it to you? He said: The Messenger of Allah (ﷺ). So he and I went, with a group of people, and I said: O Messenger of Allah, did you not teach me such and such a verse? He said: "Yes indeęd.” I said: This man claims that you taught it to him in such and such a way. The face of the Messenger of Allah (ﷺ) changed, and the man who was with him said: Let each man among you recite it as he heard it, for those who came before you were only destroyed because of differences. By Allah I do not know whether the Messenger of Allah (ﷺ) told him to say that or he said it by himself.