Musnad Abu Ya'la al-Mawsili 115:234

Ubayd Allah narrated to us, Mu'adh ibn Hisham narrated to us, my father narrated to me, from Qatadah, from Anas, that when 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 upon it." So he had a ring of silver made. It is as if I am looking at its whiteness in his hand.