Exodus 9:7, 12, 34-35

Then Pharaoh sent, and indeed, not even one of the livestock of the Israelites was dead. But the heart of Pharaoh became hard, and he did not let the people go.

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

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