The Seal of the Prophet (peace be upon him)
The Prophet (peace be upon him) being an unlettered man, could not read or write so he wore a silver ring with the words 'Muhammad Rasul Allah' which means Muhammad Messenger of God engraved in it.
He would get his scribes to write the letters and then use the seal to 'sign' them.
This is a reproduction of his seal. You'll see it at the bottom of the next letter.