Sahih Muslim | Book 24 | Hadith Number 5217
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.