Exodus 9:7, 12, 34, 35
Exodus 9:12 NIV
But the LORD hardened Pharaoh’s heart and he would not listen to Moses and Aaron, just as the LORD had said to Moses.
Exodus 9:34 NIV
When Pharaoh saw that the rain and hail and thunder had stopped, he sinned again: He and his officials hardened their hearts.
Exodus 9:35 NIV
So Pharaoh’s heart was hard and he would not let the Israelites go, just as the LORD had said through Moses.