Sahih Bukhari Volume 7, Book 72, Hadith Number 761.

Narrated By Anas bin Malik : Allah’s Apostle wanted to write a letter to a group of people or some non-Arabs. It was said to him, “They do not accept any letter unless it is stamped.” So the Prophet had a silver ring made for himself, and on it was engraved: ‘Muhammad, the Apostle of Allah’… as if I am now looking at the glitter of the ring on the finger (or in the palm) of the Prophet.