Once the Prophet ﷺ wrote a letter or had an idea of writing a letter. The Prophet ﷺ was told that they (rulers) would not read letters unless they were sealed. So the Prophet ﷺ got a silver ring made with "Muhammad Messenger of Allah ﷺ" engraved on it. As if I were just observing its white glitter in the hand of the Prophet.  

كَتَبَ النَّبِيُّ ﷺ كِتَابًا أَوْ أَرَادَ أَنْ يَكْتُبَ فَقِيلَ لَهُ إِنَّهُمْ لاَ يَقْرَءُونَ كِتَابًا إِلاَّ مَخْتُومًا فَاتَّخَذَ خَاتَمًا مِنْ فِضَّةٍ نَقْشُهُ مُحَمَّدٌ رَسُولُ اللَّهِ كَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي يَدِهِ فَقُلْتُ لِقَتَادَةَ مَنْ قَالَ نَقْشُهُ مُحَمَّدٌ رَسُولُ اللَّهِ قَالَ أَنَسٌ