Sahih Bukhari
Dress
Volume: 007 / Book Number: 072 / Hadith Number: 764
Narrated 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.