Sahih Bukhari Volume 9, Book 89, Hadith Number 276.

Narrated By Anas bin Malik : When the Prophet intended to write to the Byzantines, the people said, “They do not read a letter unless it is sealed (stamped).” Therefore the Prophet took a silver ring… as if I am looking at its glitter now… and its engraving was: “Muhammad, Apostle of Allah.”