Exodus 9:33-35

33So Moses went out of the city from Pharaoh and spread out his hands to the Lord; then the thunder and the hail ceased, and the rain was not poured on the earth.

34And when Pharaoh saw that the rain, the hail, and the thunder had ceased, he sinned yet more; and he hardened his heart, he and his servants.

35So the heart of Pharaoh was hard; neither would he let the children of Israel go, as the Lord had spoken by Moses.