Pythagoras of Samo (570 - 495 BC) was an ancient greek philosopher who became a cult leader.
However, Pythagoras' Theorem was discovered as early as the old Babylonian period that predates Pythagoras the man by about 1,000 years!
The rule was also used by the ancient Egyptian Rope Stretchers.Using ropes with equally spaced knots to form 12 equally spaced intervals (See image), they were able to construct and measure right angles.
The first recorded axiomatic proof of Pythagoras' Theorem was found in Euclid's Element Book I Prop 47.