Exodus 8:15, 31-32

But when Pharaoh saw that there was respite, he hardened his heart, and hearkened not unto them; as the LORD had said.

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