Musnad Ahmad 32:49

Isma'il narrated to us, 'Abd al-'Aziz ibn Suhayb narrated to us, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) had a ring made. He said: "We have had a ring made, and we have engraved an inscription upon it, so let no one engrave [anything] upon it."