Sunan an-Nasa'i #5201 · The Book of Adornment

Sunan an-Nasa'iIt was narrated that Anas said
Sunan an-Nasa'i · Book 48 · The Book of Adornment
Sunan an-Nasa'i · كتاب 48 · كتاب الزينة
"The Messenger of Allah [SAW] wanted to write to the Romans, but they (the Companions) said: 'They do not read any letter unless it has a seal.' So he took a ring of silver, and it is as if I can see its whiteness on his hand, and on it were engraved (the words): "Muhammad Rasul Allah (Muhammad the Messenger of Allah)
أَخْبَرَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، عَنْ بِشْرٍ، - وَهُوَ ابْنُ الْمُفَضَّلِ - قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، قَالَ أَرَادَ رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يَكْتُبَ إِلَى الرُّومِ فَقَالُوا إِنَّهُمْ لاَ يَقْرَءُونَ كِتَابًا إِلاَّ مَخْتُومًا ‏.‏ فَاتَّخَذَ خَاتَمًا مِنْ فِضَّةٍ كَأَنِّي أَنْظُرُ إِلَى بَيَاضِهِ فِي يَدِهِ وَنُقِشَ فِيهِ مُحَمَّدٌ رَسُولُ اللَّهِ ‏.‏
Reference : Sunan an-Nasa'i 5201In-book reference : Book 48, Hadith 162