Musannaf Ibn Abi Shayba 22:254

Yazid ibn Harun narrated to us, he said: Yahya ibn Sa'id informed us, from Muhammad ibn Ibrahim, from Sa'd, the freed slave of 'Amr ibn al-'As, who said: Two men disputed over an ayah and took their case up to the Messenger of Allah (may Allah bless him and grant him peace). He said: "Do not dispute regarding it, for disputing regarding it is disbelief."