The Prophet put on a silver ring with and Ethiopian stone (Fass), on which the inscription was: "Muhammad Rasul Allah."
Sunan an-Nasa'i Book 48, Hadith 238
← Previous Hadith "The inscription on the ring of the Messenger of Allah was: Muhammad Rasul Allah (Muhammad the Messenger of Allah).'"

Sunan an-Nasa'i Book 48, Hadith 237 Next Hadith → "The Messenger of Allah wanted to write to the Romans, and they (the Companions) said: 'They do not read any letter unless it has a seal.' So he took a ring of silver and it is as if I can see its whiteness on his hand, and engraved on it (were the words): 'Muhammad Rasul Allah.'"

Sunan an-Nasa'i Book 48, Hadith 239