Fighting for the Cause of Allah (Jihaad)

Bukhari :: Book 4 :: Volume 52 :: Hadith 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".