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 d

Anas reported that when Allah's Apostle (ﷺ) decided to write (letters) to non-Arabs (i. e. Persian and Byzantine Emperors) it was said to him that the non-Arabs would not accept a letter but that having a seal over it; so he (the Holy Prophet) got a silver ring made. He (Anas) said:

I perceive as if I am looking at its brightness in his hand.
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا مُعَاذُ بْنُ هِشَامٍ، حَدَّثَنِي أَبِي، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ نَبِيَّ اللَّهِ صلى الله عليه وسلم كَانَ أَرَادَ أَنْ يَكْتُبَ إِلَى الْعَجَمِ فَقِيلَ لَهُ إِنَّ الْعَجَمَ لاَ يَقْبَلُونَ إِلاَّ كِتَابًا عَلَيْهِ خَاتِمٌ ‏.‏ فَاصْطَنَعَ خَاتَمًا مِنْ فِضَّةٍ ‏.‏ قَالَ كَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي يَدِهِ ‏.‏
Reference : Sahih Muslim 2092d
In-book reference : Book 37, Hadith 96
USC-MSA web (English) reference : Book 24, Hadith 5217
  (deprecated numbering scheme)