Sahih al-Bukhari, Volume 4, Book 52, Hadith 189 (USC-MSA) ⓘ

Narrated Anas: When the Prophet intended to write a letter to the ruler of the Byzantines, he was told that those people did not read any letter unless it was stamped with a seal. So, the Prophet got a silver ring-- as if I were just looking at its white glitter on his hand ---- and stamped on it the expression "Muhammad, Apostle of Allah".

حَدَّثَنَا عَلِيُّ بْنُ الْجَعْدِ، أَخْبَرَنَا شُعْبَةُ، عَنْ قَتَادَةَ، قَالَ سَمِعْتُ أَنَسًا ـ رضى الله عنه ـ يَقُولُ لَمَّا أَرَادَ النَّبِيُّ صلى الله عليه وسلم أَنْ يَكْتُبَ إِلَى الرُّومِ، قِيلَ لَهُ إِنَّهُمْ لاَ يَقْرَءُونَ كِتَابًا إِلاَّ أَنْ يَكُونَ مَخْتُومًا‏.‏ فَاتَّخَذَ خَاتَمًا مِنْ فِضَّةٍ، فَكَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي يَدِهِ، وَنَقَشَ فِيهِ مُحَمَّدٌ رَسُولُ اللَّهِ‏.‏