Anas Radiyallahu relates that:
Rasulullah sallallahu alayhi wasallam made an intention to write letters to Kisra, Qaysur (Ceasar) and Najashi, inviting them to accept Islaam. The people said: '(O Rasulullah) those people do not accept letters without a stamp on it'. For this reason Rasulullah Sallallahu Alayhi Wasallam had a stamp made. The ring (loop) of which was silver, and had 'Muhammad Rasulullah' engraved on it".
English reference: Book 11, Hadith 87
Arabic reference: Book 12, Hadith 92