From Sahih Bukhari (Hadith 7162):
Narrated Anas bin Malik:When the Prophet (ﷺ) intended to write to the Byzantines, the people said, "They do not read a letter unless it is sealed (stamped)." Therefore the Prophet (ﷺ) took a silver ring----as if I am looking at its glitter now----and its engraving was: 'Muhammad, Apostle of Allah
Reference: Bukhari 7162