(47) باب صِفَةِ خَاتَمِ النَّبِيِّ صلى الله عليه وسلم
It was narrated that Anas said:
"The Messenger of Allah [SAW] 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 reference: Book 48, Hadith 162
English translation: Vol. 6, Book 48, Hadith 5204