Sahih Muslim | Book 24 | Hadith Number 5218

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).