Musnad Ahmad 32:1103

Muhammad ibn Bishr narrated to us, Sa'id ibn Abi 'Aruba narrated to us, from Qatadah, from Anas ibn Malik, that he narrated to them: "The Messenger of Allah (peace and blessings of Allah be upon him) intended to write to the non-Arabs. It was said: 'They do not accept a letter unless it has a seal.' So the Messenger of Allah (peace and blessings of Allah be upon him) took a ring of silver, and he had engraved upon it: 'Muhammad is the Messenger of Allah.'"