Sahih Bukhari Volume 007, Book 072, Hadith Number 767.
Narrated By Anas : 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. Anas added: 'the ring of the Prophet 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.