"The Messenger of Allah [SAW] 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.'"
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