Exodus 9:33-37

And Moses went out of the city from Pharaoh, and spread abroad his hands unto the LORD: and the thunders and hail ceased, and the rain was not poured upon the earth.

And when Pharaoh saw that the rain and the hail and the thunders were ceased, he sinned yet more, and hardened his heart, he and his servants.

And the heart of Pharaoh was hardened, neither would he let the children of Israel go; as the LORD had spoken by Moses.