Sahih Muslim , Book 24, Hadith 5216 (USC-MSA) ⓘ

Anas b. Malik reported that when Allah's Messenger decided to write letters to the Byzantine (Emperor) they (his Companions) told him that they would not read a letter unless it is sealed. (Then) Allah's Messenger had a silver ring made (for himself), (its shape is so vivid in my mind) as if I see its brightness in the band of Allah's Messenger and its engravement was (Muhammad, Messenger of Allah).

حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالَ ابْنُ الْمُثَنَّى حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، قَالَ سَمِعْتُ قَتَادَةَ، يُحَدِّثُ عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ لَمَّا أَرَادَ رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يَكْتُبَ إِلَى الرُّومِ - قَالَ - قَالُوا إِنَّهُمْ لاَ يَقْرَءُونَ كِتَابًا إِلاَّ مَخْتُومًا ‏.‏ قَالَ فَاتَّخَذَ رَسُولُ اللَّهِ صلى الله عليه وسلم خَاتَمًا مِنْ فِضَّةٍ كَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي يَدِ رَسُولِ اللَّهِ صلى الله عليه وسلم نَقْشُهُ مُحَمَّدٌ رَسُولُ اللَّهِ ‏.‏