The Prophet wore a ring of silver with an Ethiopian stone (Fass), on which was inscribed (the phrase): "Muhammad Rasul Allah (Muhammad the Messenger of Allah)."
Sunan an-Nasa'i Book 48, Hadith 157
← Previous Hadith A man came to the Prophet and he was wearing an iron ring. He said: "Why do I see you wearing the jewelry of the people of Hell?" He threw it away, then he came and he was wearing a brass ring. He said: "Why do I notice the stench of idols from you?" So he threw it away, and said: "O Messenger of Allah, what should I use?" He said: "Silver, but it should not equal a Mithqal."

Sunan an-Nasa'i Book 48, Hadith 156 Next Hadith → "The Messenger of Allah had a ring of silver which he wore on his right hand. It had an Ethiopian stone (Fass), and he used to wear the stone (Fass) next to his palm."

Sunan an-Nasa'i Book 48, Hadith 158