(101) باب دَعْوَةِ الْيَهُودِيِّ وَالنَّصْرَانِيِّ، وَعَلَى مَا يُقَاتَلُونَ عَلَيْهِ
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". (ES)
Reference (ES): Sahih al-Bukhari 2938 (ES)
In-book reference (ES): Book 56, Hadith 151 (ES)
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 4, Book 52, Hadith 189 (ES)