Exodus 9:33-35

33Moses left Pharaoh and the city, and spread out his hands to the Lord. Then the thunder and hail ceased, and rain no longer poured down on the land.

34When Pharaoh saw that the rain, hail, and thunder had ceased, he sinned again and hardened his heart, he and his officials.

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