Sahih Bukhari Volume 007, Book 072, Hadith Number 764.

Narrated By 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.