Musnad Ahmad 32:704

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Thabit al-Bunani, from Anas ibn Malik: "The Prophet, peace and blessings of Allah be upon him, made a ring of silver, and he engraved upon it 'Muhammad is the Messenger of Allah.' Then he said: 'Do not engrave [anything] upon it.'"