Anas radiallahu anhu relates:
"When Rasulullah sallallahu alayhe wasallam intended to write letters to the kings of 'Ajam (non arabs), inviting them to Islam, the people said that they did not accept letters without a seal (stamp) on them. Rasulullah sallallahu alayhe wasallam therefore had a ring made, the whiteness of which is still before my eyes. (ES)
English reference (ES): Book 11, Hadith 85 (ES)
Arabic reference (ES): Book 12, Hadith 90 (ES)