Anas said that when the Prophet (ﷺ) wanted to write to the Persian Emperor, the Byzantine Emperor and the Negus he was told that they would not accept a letter without a seal; so God’s messenger fashioned a seal in the form of a silver ring on which he engraved “Muhammad God’s messenger.” Muslim transmitted it. A version by Bukhari says the engraving on the seal was in three lines, “Muhammad” being one, “messenger” another, and “of God” another.
Source: sunnah.com
Other translations
Arabic
وَعَنْ أَنَسٍ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَرَادَ أَنْ يَكْتُبَ إِلَى كِسْرَى وَقَيْصَرَ وَالنَّجَاشِيِّ فَقِيلَ: إِنَّهُمْ لَا يَقْبَلُونَ كِتَابًا إِلَّا بِخَاتَمٍ فَصَاغَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ خَاتَمًا حَلْقَةَ فِضَّةٍ نُقِشَ فِيهِ: مُحَمَّدٌ رَسُولُ اللَّهِ. رَوَاهُ مُسْلِمٌ. وَفِي رِوَايَةٍ لِلْبُخَارِيِّ: كَانَ نَقْشُ الْخَاتَمِ ثَلَاثَةَ أَسْطُرٍ: مُحَمَّدٌ سَطْرٌ ورسولُ الله سطر وَالله سطر
sunnah.com