Musnad Ahmad 32:921

Waki’ narrated to us, from Shu’bah, from Qatadah; and Ibn Ja’far narrated to us, Shu’bah said: I heard Qatadah—the meaning—from Anas: "That the Prophet, may Allah bless him and grant him peace, wanted to write a letter to the Romans. It was said to him: 'If it is not sealed, they will not read your letter.' So he took a ring of silver, and engraved upon it: 'Muhammad the Messenger of Allah.' It is as if I am looking at its whiteness in his palm."