This result was known by the ancient Babylonians as early as $2000$ BCE.
The proof of what is now known as Thales' Theorem that Thales of Miletus actually used is unknown. Whether he was aware of the Interior Angles Theorem and used it in his proof cannot be decided.
The Interior Angles Theorem is traditionally ascribed to Pythagoras, who lived near Thales, and may have met him.
Thus it is possible that Pythagoras learned of the Interior Angles Theorem either directly or indirectly from Thales himself.