Sahih Muslim Book 24, Hadith Number 5212.

Chapter : Wearing of silver ring by Allah’s Apostle (may peace he upon him) in which were engraved the words (Muhammad the Messenger of Allah) and after him his successors wore it.

Ibn Umar reported that Allah’s Messenger (may peace be upon him) had made for himself a ring of silver, and he (wore it in his finger), then it was in Abu Bakr’s finger, then it was in ‘Umar’s finger, then it was in ‘Uthman’s finger, until it fell into the well of Aris and it had these words engraved upon it (Muhammad, Messenger of Allah). Ibn Numair narrated it with a slight variation of words.