كتاب اللباس والزينة
37  
The Book of Clothes and Adornment
(13)
Chapter: The Prophet (SAW) Acquired A Ring When He Wanted To Send Letters To The Non-Arabs
(13)
باب فِي اتِّخَاذِ النَّبِيِّ صلى الله عليه وسلم خَاتَمًا لَمَّا أَرَادَ أَنْ يَكْتُبَ إِلَى الْعَجَمِ ‏‏
Sahih Muslim 2092 e

Anas reported that when Allah's Apostle (ﷺ) decided to write to the Kisra (the King of Persia), Caesar (Emperor of Rome), and the Negus (the Emperor of Abyssinia), it was said to him that they would not accept the letter without the seal over it; so Allah's Messenger (ﷺ) got a seal made, the ring of which was made of silver and there was engraved on it:

"Muhammad, the Messenger of Allah."
حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، حَدَّثَنَا نُوحُ بْنُ قَيْسٍ، عَنْ أَخِيهِ، خَالِدِ بْنِ قَيْسٍ عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم أَرَادَ أَنْ يَكْتُبَ إِلَى كِسْرَى وَقَيْصَرَ وَالنَّجَاشِيِّ ‏.‏ فَقِيلَ إِنَّهُمْ لاَ يَقْبَلُونَ كِتَابًا إِلاَّ بِخَاتِمٍ ‏.‏ فَصَاغَ رَسُولُ اللَّهِ صلى الله عليه وسلم خَاتَمًا حَلَقَةً فِضَّةً وَنَقَشَ فِيهِ مُحَمَّدٌ رَسُولُ اللَّهِ ‏.‏
Reference : Sahih Muslim 2092e
In-book reference : Book 37, Hadith 97
USC-MSA web (English) reference : Book 24, Hadith 5218
  (deprecated numbering scheme)