Sahih al-Bukhari 7162
Sahih al-Bukhari · Book 93: Judgments (Ahkaam) · To bear witness as to the writer of a stamped letter; and the letter of a ruler to a governor, and of a judge to a judge
Narrated Anas bin Malik:
حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ ، حَدَّثَنَا غُنْدَرٌ ، حَدَّثَنَا شُعْبَةُ ، قَالَ سَمِعْتُ قَتَادَةَ ، عَنْ أَنَسِ بْنِ مَالِكٍ ، قَالَ لَمَّا أَرَادَ النَّبِيُّ صلى الله عليه وسلم أَنْ يَكْتُبَ إِلَى الرُّومِ قَالُوا إِنَّهُمْ لاَ يَقْرَءُونَ كِتَابًا إِلاَّ مَخْتُومًا. فَاتَّخَذَ النَّبِيُّ صلى الله عليه وسلم خَاتَمًا مِنْ فِضَّةٍ، كَأَنِّي أَنْظُرُ إِلَى وَبِيصِهِ، وَنَقْشُهُ مُحَمَّدٌ رَسُولُ اللَّهِ.
When the Prophet (ﷺ) intended to write to the Byzantines, the people said, "They do not read a letter unless it is sealed (stamped)." Therefore the Prophet (ﷺ) took a silver ring----as if I am looking at its glitter now----and its engraving was: 'Muhammad, Apostle of Allah'.