Musnad Abu Ya'la al-Mawsili 130:49

Ishaq narrated to us, Hammad narrated to us, from Abd al-Aziz, from Anas, that the Prophet (peace be upon him) took a ring of silver and engraved upon it "Muhammad is the Messenger of Allah." He said to the people, "I have taken a ring and its engraving is 'Muhammad,' so let no one engrave upon its engraving."