Exodus 9:7, 12, 34-35

And Pharaoh sent, and behold, not one of the livestock of Israel was dead. But the heart of Pharaoh was hardened, and he did not let the people go.

But the LORD hardened the heart of Pharaoh, and he did not listen to them, as the LORD had spoken to Moses.

But when Pharaoh saw that the rain and the hail and the thunder had ceased, he sinned yet again and hardened his heart, he and his servants. So the heart of Pharaoh was hardened, and he did not let the people of Israel go, just as the LORD had spoken through Moses.