Musnad Ahmad 32:777

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, and Hajjaj narrated to us, he said: Shu'bah narrated to me, he said: I heard Qatadah narrating from Anas ibn Malik, who said: When the Messenger of Allah, peace and blessings of Allah be upon him, wanted to write to the Romans, they said: "They do not read a letter unless it is sealed." He said: "So the Messenger of Allah, peace and blessings of Allah be upon him, took a ring of silver. It is as if I am looking at its whiteness in the hand of the Messenger of Allah, peace and blessings of Allah be upon him, its engraving was: 'Muhammad is the Messenger of Allah'."