Sahih Muslim | Book 24 | Chapter 12

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).

Narrated by Anas
Anas reported that when Allah's Apostle (may peace be upon him) decided to write (letters) to non-Arabs (i.e. Persian and Byzantine Emperors) it was said to him that the non-Arabs would not accept a letter but that having a seal over it; so he (the Holy Prophet) got a silver ring made. He (Anas) said: I perceive as if I am looking at its brightness in his hand.

Narrated by Anas
Anas reported that when Allah's Apostle (may peace be upon him) decided to write to the Kisri (the King of Persia), Caesar (Emperor of Rome), and the Negus (the Emperor of Abyssinia), it was said to him that they would not accept the letter without the seal over it; so Allah's Messenger (may peace be upon him) got a seal made, the ring of which was made of silver and there was engraved on it. (Muhammad. the Messenger of Allab).