Sahih Muslim
The Book Pertaining to Clothes and Decoration (Kitab Al-Libas wal-Zinah)
Book Number: 024 / Hadith Number: 5216
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 himealf), (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 (Mubamrnad, Messenger of Allah).