Narrated Anas bin Malik: Once the Prophet (peace be upon him) 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

Bukhari Volume 1 Book 3 No. 65