It is reported that the Prophet ﷺ wanted to write letters to Khosrow, Caesar, and the Negus. He was told that they would not accept any letter unless it bore a seal. So the Messenger of Allah ﷺ had a ring made with a silver band, and engraved upon it, "Muhammad, Messenger of Allah."
أن النبي ﷺ أراد أن يكتب إلى كسرى، وقيصر، والنجاشي، فقيل: إنهم لا يقبلون كتابا إلا بخاتم، فصاغ رسول الله ﷺ خاتما حلقته فضة، ونقش فيه: محمد رسول الله.