Muslim 5480

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)