Sahih Muslim , Book 24, Hadith 5217 (USC-MSA) ⓘ

Anas reported that when Allah's Apostle 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 ‏.‏