Sahih Bukhari | Volume 7 | Hadith Number 764

Narrated by Anas bin Malik
When the Prophet intended to write to the Byzantines, it was said to him, "Those people do not read your letter unless it is stamped." So the Prophet took a silver ring and got 'Muhammad, the Apostle of Allah' engraved on it... as if I am now looking at its glitter in his hand.