Sahih Muslim
The Book Pertaining to Clothes and Decoration (Kitab Al-Libas wal-Zinah)
Book Number: 024 / Hadith Number: 5217
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.