"The Messenger of Allah (ﷺ) wanted to write to the Romans, and they (the Companions) said: 'They do not read any letter unless it has a seal.' So he took a ring of silver and it is as if I can see its whiteness on his hand, and engraved on it (were the words): 'Muhammad Rasul Allah.'"
Reference: Sunan an-Nasa'i 5278 In-book : Book 48, Hadith 239 English translation : Vol. 6, Book 48, Hadith 5280
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