Musnad Ahmad 26:254

'Abd al-Rahman narrated to us, from Hammam, from 'Asim, from Abu Wa'il, from 'Abdullah, who said: I heard a man reciting the Ha-Mim of the thirty—meaning al-Ahqaf—and he recited a letter, and another man recited a letter that his companion did not recite, and I recited letters that my companion did not recite. So we went to the Prophet, may Allah bless him and grant him peace, and informed him. He said: "Do not differ, for those who were before you were only destroyed by their differing." Then he said: "Look to the one among you who is the most proficient in recitation, and take from his recitation."