Fighting for the Cause of Allah (Jihaad)
Bukhari :: Book 4 :: Volume 52 :: Hadith 189
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".
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