It was narrated that Anas said:

"The Messenger of Allah ﷺ wanted to write to the Romans, and they (the Companions) said: 'They do not read any letter unless it has a seal.' So he took a ring of silver and it is as if I can see its whiteness on his hand, and engraved on it (were the words): 'Muhammad Rasul Allah.'"

أَخْبَرَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، عَنْ بِشْرٍ، - وَهُوَ ابْنُ الْمُفَضَّلِ - قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، قَالَ أَرَادَ رَسُولُ اللَّهِ ﷺ أَنْ يَكْتُبَ إِلَى الرُّومِ فَقَالُوا إِنَّهُمْ لاَ يَقْرَءُونَ كِتَابًا إِلاَّ مَخْتُومًا . فَاتَّخَذَ خَاتَمًا مِنْ فِضَّةٍ كَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي يَدِهِ وَنُقِشَ فِيهِ مُحَمَّدٌ رَسُولُ اللَّهِ .