Bukhari :: Book 7 :: Volume 72 :: Hadith 761
Narrated 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 .
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