Musnad Abu Ya'la al-Mawsili 116:58

Ahmad narrated to us, Shababa narrated to us, Shu'bah narrated to us, from Qatadah, from Anas, who said: When the Prophet, may Allah bless him and grant him peace, wanted to write to the Romans, it was said to him: "They will not read a letter unless it is sealed." So he took a ring of silver, and its engraving was "Muhammad the Messenger of Allah." Anas said: It is as if I am looking at its whiteness in his hand.