Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith 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".