Sahih Muslim , Book 37, Hadith 97 (In-Book) ⓘ

Anas reported that when Allah's Apostle 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 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 ‏.‏