Sahih al-Bukhari 2938
Sahih al-Bukhari · Book 56: Fighting for the Cause of Allah (Jihaad) · (What is said regarding the invitation of the Jews and Christians (to Islam), and for what Muslims should declare war against them and what the Prophet ﷺ wrote to Khusrau and Caesar
Narrated Anas:
حَدَّثَنَا عَلِيُّ بْنُ الْجَعْدِ ، أَخْبَرَنَا شُعْبَةُ ، عَنْ قَتَادَةَ ، قَالَ سَمِعْتُ أَنَسًا ـ رضى الله عنه ـ يَقُولُ لَمَّا أَرَادَ النَّبِيُّ صلى الله عليه وسلم أَنْ يَكْتُبَ إِلَى الرُّومِ، قِيلَ لَهُ إِنَّهُمْ لاَ يَقْرَءُونَ كِتَابًا إِلاَّ أَنْ يَكُونَ مَخْتُومًا. فَاتَّخَذَ خَاتَمًا مِنْ فِضَّةٍ، فَكَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي يَدِهِ، وَنَقَشَ فِيهِ مُحَمَّدٌ رَسُولُ اللَّهِ.
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".