Sahih Bukhari Volume 001, Book 003, Hadith Number 065.

Narated By Anas bin Malik : Once the Prophet wrote a letter or had an idea of writing a letter. The Prophet was told that they (rulers) would not read letters unless they were sealed. So the Prophet got a silver ring made with "Muhammad Allah's Apostle" engraved on it. As if I were just observing its white glitter in the hand of the Prophet.