"The Messenger of Allah (ﷺ) wanted to write to the Romans, but 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 on it were engraved (the words): "Muhammad Rasul Allah (Muhammad the Messenger of Allah)."
Reference: Sunan an-Nasa'i 5201 In-book : Book 48, Hadith 162 English translation : Vol. 6, Book 48, Hadith 5204
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