Exodus 9:33-37

So Moses left the city from his meeting with Pharaoh, and spread out his hands to the Lord; and the thunder and the hail stopped, and rain no longer poured on the earth.

But when Pharaoh saw that the rain and the hail and the thunder had stopped, he sinned again and hardened his heart, he and his servants.

So Pharaoh’s heart was hardened, and he did not let the sons of Israel go, just as the Lord had spoken through Moses.