Exodus 9:7, 12, 34-35

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

And the LORD hardened Pharaoh’s heart, and he did not listen to them, just as the LORD had spoken to Moses.

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