that when Abu Bakr became the Caliph, he wrote a letter to him (and stamped it with the Prophet's ring) and the engraving of the ring was in three lines: Muhammad in one line, 'Apostle' in another line, and 'Allah' in a third line.
Sahih al-Bukhari Book 77, Hadith 95
← Previous Hadith Allah's Messenger (ﷺ) took a silver ring and had 'Muhammad, the Apostle' of Allah' engraved on it. The Prophet then said (to us), 'I have a silver ring with 'Muhammad, the Messenger of Allah engraved on it, so none of you should have the same engraving on his ring."

Sahih al-Bukhari Book 77, Hadith 94 Next Hadith → The ring of the Prophet (ﷺ) was in his hand, and after him, in Abu Bakr's hand, and then in `Umar's hand after Abu Bakr. When `Uthman was the Caliph, once he was sitting at the well of Aris. He removed the ring from his hand and while he was trifling with it, dropped into the well. We kept on going to the well with `Uthman for three days looking for the ring, and finally the well was drained, but the ring was not found.

Sahih al-Bukhari Book 77, Hadith 96