When the Prophet ﷺ intended to write to the Romans, he was told, "They will not read your letter unless it is sealed."So he had a ring made of silver, engraved with "Muhammad, Messenger of Allah." It is as if I can still see its whiteness on his hand.
لما أراد النبي ﷺ أن يكتب إلى الروم قيل له: إنهم لن يقرؤوا كتابك إذا لم يكن مختوما، فاتخذ خاتما من فضة، ونقشه: محمد رسول الله، فكأنما أنظر إلى بياضه في يده.
Bukhari narrated it in the Book of Dress (5875), and Muslim in the Book of Dress (2092:56), both of them via Shubah, who said, "I heard Qatada narrating from Anas bin Malik," and he mentioned it.