Musnad Ahmad 32:1972

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, he said: "When the Messenger of Allah, peace and blessings of Allah be upon him, wanted to write a letter to the Romans, they said: 'They do not read a letter unless it is sealed.' 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.'"