Sahih Bukhari Volume 4, Book 52, Hadith Number 189.

Narated By Anas : When the Prophet intended to write a letter to the ruler of the Byzantines, he was told that those people did not read any letter unless it was stamped with a seal. So, the Prophet got a silver ring… as if I were just looking at its white glitter on his hand… and stamped on it the expression “Muhammad, Apostle of Allah”.