Musnad Ahmad 32:1383

Hashim narrated to us, Shu'bah narrated to us, he said: Qatadah informed me, from Anas bin Malik, who said: When the Messenger of Allah (ﷺ) wanted to write to the Romans, it was said to him: "Your letter will not be read unless it is sealed." So he took a ring of silver, and he had it engraved, or [he had] "Muhammad the Messenger of Allah" engraved [upon it]. He said: "It is as if I am looking at its whiteness in his hand."