Sahih Bukhari
Judgments (Ahkaam)
Volume: 009 / Book Number: 089 / Hadith Number: 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'