What was the original proof that Pythagoras himself used to prove his theorem?
We don't know.
We don't have any of Pythagoras original writings.
We only have hearsay from writers of later centuries that Pythagoras did any significant mathematics, though his followers were significantly interested in mathematics.
According to later writers, Pythagoras (or one of his followers) found the
Pythagoras Theorem was known to the Babylonians (and others) 1000 or so years before Pythagoras, and it seems likely that they had a proof, though we have not identified one yet in their cuneiform writings.