When the Prophet ﷺ intended to write to the Byzantines, the people said, "They do not read a letter unless it is sealed (stamped)." Therefore the Prophet ﷺ took a silver ring----as if I am looking at its glitter now----and its engraving was: 'Muhammad, Apostle of Allah'.
لَمَّا أَرَادَ النَّبِيُّ ﷺ أَنْ يَكْتُبَ إِلَى الرُّومِ قَالُوا إِنَّهُمْ لاَ يَقْرَءُونَ كِتَابًا إِلاَّ مَخْتُومًا فَاتَّخَذَ النَّبِيُّ ﷺ خَاتَمًا مِنْ فِضَّةٍ كَأَنِّي أَنْظُرُ إِلَى وَبِيصِهِ وَنَقْشُهُ مُحَمَّدٌ رَسُولُ اللَّهِ