He said God’s messenger wore on his right hand a silver signet-ring with an Abyssinian stone, and put its stone next the palm of his hand.
(Bukhari and Muslim.)
Anas said that when the Prophet wanted to write to the Persian Emperor, the Byzantine Emperor and the Negus he was told that they would not accept a letter without a seal; so God’s messenger fashioned a seal in the form of a silver ring on which he engraved “Muhammad God’s messenger.”
Muslim transmitted it. A version by Bukhari says the engraving on the seal was in three lines, “Muhammad” being one, “messenger” another, and “of God” another.