Sahih al-Bukhari — Fighting for the Cause of Allah (Jihaad) — Hadith No: 2938
Translation
Narrated Anas:When the Prophet (ﷺ) 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 (ﷺ) 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