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.
English reference: Book 11, Hadith 85
Arabic reference: Book 12, Hadith 90