Bukhari :: Book 7 :: Volume 72 :: Hadith 764

Narrated Anas bin Malik:

When the Prophet intended to write to the Byzantines, it was said to him, "Those people do not read your letter unless it is stamped." So the Prophet took a silver ring and got 'Muhammad, the Apostle of Allah' engraved on it .... as if I am now looking at its glitter in his hand.

Source materials are from the University of Southern California MSA site
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