Exodus 8:15, 31-32

But when Pharaoh saw that there was a respite, he hardened his heart and would not listen to them, as the LORD had said.

And the LORD did as Moses asked, and removed the swarms of flies from Pharaoh, from his servants, and from his people; not one remained. But Pharaoh hardened his heart this time also, and did not let the people go.