(15) باب الشَّهَادَةِ عَلَى الْخَطِّ الْمَخْتُومِ
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: Sahih al-Bukhari 7162
In-book reference: Book 93, Hadith 26
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 9, Book 89, Hadith 276