(1) باب مَا جَاءَ فِي اتِّخَاذِ الْخَاتَمِ
Narrated Anas bin Malik:
The Messenger of Allah (ﷺ) wanted to write to some persian rulers. He was told that they would not read a letter without a seal in the form of a silver ring on which he engraved "Muhammad the Messenger of Allah."
Reference: Sunan Abi Dawud 4214
In-book reference: Book 36, Hadith 1
English translation: Book 35, Hadith 4202