Volume 4, Book 52, Number 189:
Narrated 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”.