Judgments (Ahkaam)

Bukhari :: Book 9 :: Volume 89 :: Hadith 276

Narrated 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'