Musnad Abu Ya'la al-Mawsili 130:9

Abu al-Rabi' narrated to us, Hammad narrated to us, Abd al-Aziz narrated to us, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) took a ring of gold and engraved upon it "Muhammad is the Messenger of Allah," and he forbade that anyone should engrave upon its engraving.