Musnad Ahmad 32:795

Muhammad ibn Ja'far narrated to us, Sa'id and Muhammad ibn Bakr narrated to us, he said: Sa'id informed us, from Qatadah, from Anas ibn Malik, that when the Messenger of Allah (peace and blessings of Allah be upon him) wanted to write to some of these non-Arabs, it was said to him: "They do not accept a letter unless it has a seal." He said: "So he took a ring of silver, its engraving..." And Ibn Bakr said: "...and its engraving was 'Muhammad the Messenger of Allah'. It is as if I am looking at its glitter - or its shine - in the hand of the Messenger of Allah (peace and blessings of Allah be upon him)."