Sahih Muslim | Book 24 | Hadith Number 5216

Narrated by Anas b. Malik
Anas b. Malik reported that when Allah's Messenger (may peace be upon him) 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 (may peace be upon him) had a silver ring made (for himself), (its shape is to vivid in my mind) as if I see its brightness in the band of Allah's Messenger (may peace be upon him) and its engravement was (Muhammad, Messenger of Allah).