(13) باب فِي اتِّخَاذِ النَّبِيِّ صلى الله عليه وسلم خَاتَمًا لَمَّا أَرَادَ أَنْ يَكْتُبَ إِلَى الْعَجَمِ
Anas b. Malik reported that when Allah's Messenger (ﷺ) 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 (ﷺ) 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 (ﷺ) and its engravement was (Muhammad, Messenger of Allah).
Reference: Sahih Muslim 2092 c
In-book reference: Book 37, Hadith 95
USC-MSA web (English) reference(deprecated numbering scheme): Book 24, Hadith 5216