Definition:Pythagorean Triangle/Historical Note
Historical Note on Pythagorean Triangle
Pythagorean triangles were known to the ancient Babylonians in about $2000$ BCE.
It is also clear that the author of that tablet was also familiar with the fact, proved in Solutions of Pythagorean Equation, that the numbers $2 p q$, $p^2 - q^2$ and $p^2 + q^2$ always make a Pythagorean triangle.
It is likely that the ancient Greeks, and in particular Pythagoras or one of his disciples, obtained this result from the Babylonians.