Musannaf Ibn Abi Shayba 3:760

Abu al-Ahwas narrated to us, from Ash'ath ibn Abi al-Sha'tha', from his father, who said: I was sitting with Ibn Umar when he saw a man who had the mark of prostration on his face. He said: "The image of a man is his face, so let none of you disfigure his image."