From Sahih Bukhari (Hadith 5872):
Narrated Anas bin Malik:Allah's Messenger (ﷺ) 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 (ﷺ) had a silver ring made for himself, and on it was engraved: 'Muhammad, the Messenger of Allah'. .. as if I am now looking at the glitter of the ring on the finger (or in the palm) of the Prophet (ﷺ)
Reference: Bukhari 5872