From Sahih Bukhari (Hadith 65):
Narrated Anas bin Malik:Once the Prophet (ﷺ) wrote a letter or had an idea of writing a letter. The Prophet (ﷺ) was told that they (rulers) would not read letters unless they were sealed. So the Prophet (ﷺ) got a silver ring made with "Muhammad Allah's Messenger (ﷺ)" engraved on it. As if I were just observing its white glitter in the hand of the Prophet
Reference: Bukhari 65