Sahih Muslim Book 24, Hadith Number 5213.

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 Apostle (may peace be upon him) had made for himself a gold ring; then lie discarded it, and then made for himself a silver ring and had these words engraved upon it (Muhammad, Messenger of Allah), and said: No one should engrave anything like the engraving of this signet ring of mine and when he wore it, he kept its stone towards the inside of his palm, and it was this which fell down (from the hands) of Mu’ayqib into the well of Aris.