Sahih Bukhari
Sahih Bukhari Volume 009, Book 089, Hadith Number 276.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 89. Judgments (Ahkaam)
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."