Musnad Abu Ya'la al-Mawsili 115:168

Abu Musa narrated to us, Mu'adh ibn Hisham narrated to us, my father narrated to me, from Qatadah, from Anas, that the Prophet of Allah (may Allah bless him and grant him peace) wanted to write to the non-Arabs. It was said to him, "The non-Arabs do not accept a letter unless it has a seal." So, the Messenger of Allah (may Allah bless him and grant him peace) had a ring of silver made. It is as if I am looking at its whiteness on his hand.