Sunan an-Nasa'i 5278
سنن النسائي#5278SahihIt was narrated that Anas said:"The Messenger of Allah [SAW] 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
In-book reference: 48:239