Exodus 9:7, 12, 34-35

And Pharaoh sent, and, behold, there was not so much as one of the cattle of the Israelites dead. But the heart of Pharaoh was stubborn, and he did not let the people go.

And Jehovah hardened the heart of Pharaoh, and he hearkened not unto them, as Jehovah had spoken unto Moses.

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, and he did not let the children of Israel go, as Jehovah had spoken by Moses.