Musnad Abu Ya'la al-Mawsili 116:57

Ahmad narrated to us, Wahb ibn Jarir narrated to us, Shu'bah narrated to us, from Qatadah, from Anas, that the Prophet (peace and blessings of Allah be upon him) wrote to the Romans. It was said to him, "They do not accept a letter unless it has a seal." So he took a ring of silver. It is as if I am looking at its whiteness in his hand, its engraving being: "Muhammad is the Messenger of Allah."