Exodus 9:33-35

33Then Moses left Pharaoh and went out of the city. He spread out his hands toward the Lord; the thunder and hail stopped, and the rain no longer poured down on the land.

34When Pharaoh saw that the rain and hail and thunder had stopped, he sinned again: He and his officials hardened their hearts.

35So Pharaoh’s heart was hard and he would not let the Israelites go, just as the Lord had said through Moses.