Musnad Ahmad 32:998

Yunus narrated to us, Hammad—meaning Ibn Zayd—narrated to us, Abd al-Aziz ibn Suhayb narrated to us, from Anas ibn Malik, that the Messenger of Allah (may Allah bless him and grant him peace) took a ring of silver, and he had engraved upon it: "Muhammad is the Messenger of Allah." The Messenger of Allah (may Allah bless him and grant him peace) said: "I have taken a ring of silver and engraved upon it 'Muhammad is the Messenger of Allah,' so do not engrave [anything] upon it."