Exodus 9:7, 12, 34-35

Then Pharaoh sent [men to investigate], and not even one of the livestock of the Israelites had died. But the heart of Pharaoh was hardened [and his mind was firmly set], and he did not let the people go.

But the LORD hardened the heart of Pharaoh, and he did not listen or pay attention to them, just as the LORD had told Moses.

But when Pharaoh saw that the rain and the hail and the thunder had ceased, he sinned again and hardened his heart, both he and his servants. Pharaoh’s heart was hardened, and he did not let the Israelites go, just as the LORD had said through Moses. [Ex 4:21]