Anas reported that when Allah's Apostle ﷺ decided to write to the Kisri (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 Messenger of Allah ﷺ got a seal made, the ring of which was made of silver and there was engraved on it. (Mubammad. the Messenger of Allab).

حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، حَدَّثَنَا نُوحُ بْنُ قَيْسٍ، عَنْ أَخِيهِ، خَالِدِ بْنِ قَيْسٍعَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ النَّبِيَّ ﷺ أَرَادَ أَنْ يَكْتُبَ إِلَى كِسْرَى وَقَيْصَرَ وَالنَّجَاشِيِّ. فَقِيلَ إِنَّهُمْ لاَ يَقْبَلُونَ كِتَابًا إِلاَّ بِخَاتِمٍ . فَصَاغَ رَسُولُ اللَّهِ ﷺ خَاتَمًاحَلَقَةً فِضَّةً وَنَقَشَ فِيهِ مُحَمَّدٌ رَسُولُ اللَّهِ .