Sahih al-Bukhari 65Book 3, Hadith 7
حَدَّثَنَا مُحَمَّدُ بْنُ مُقَاتِلٍ أَبُو الْحَسَنِ، أَخْبَرَنَا عَبْدُ اللَّهِ، قَالَ أَخْبَرَنَا شُعْبَةُ، عَنْ قَتَادَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ كَتَبَ النَّبِيُّ صلى الله عليه وسلم كِتَابًا ـ أَوْ أَرَادَ أَنْ يَكْتُبَ ـ 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.
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 Allah's Messenger (ﷺ)" engraved on it. As if I were just observing its white glitter in the hand of the Prophet.