Exodus 8:15, 31-32

But when Pharaoh saw that there was relief, he hardened his heart and did not heed them, as the LORD had said.

And the LORD did according to the word of Moses; He removed the swarms of flies from Pharaoh, from his servants, and from his people. Not one remained. But Pharaoh hardened his heart at this time also; neither would he let the people go.