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 ﷺ كَانَ أَرَادَ أَنْ يَكْتُبَ إِلَى الْعَجَمِ فَقِيلَ لَهُ إِنَّ الْعَجَمَ لاَ يَقْبَلُونَإِلاَّ كِتَابًا عَلَيْهِ خَاتِمٌ . فَاصْطَنَعَ خَاتَمًا مِنْ فِضَّةٍ . قَالَ كَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي يَدِهِ.