Exodus 4:21
And the LORD said to Moses, When you go to return into Egypt, see that you do all those wonders before Pharaoh, which I have put in your hand: but I will harden his heart, that he shall not let the people go.
Nave's Topical Index—AKJV

Exodus 7:3,13,22
And I will harden Pharaoh's heart, and multiply my signs and my wonders in the land of Egypt.
Nave's Topical Index—AKJV

Exodus 8:15,32
But when Pharaoh saw that there was respite, he hardened his heart, and listened not to them; as the LORD had said.
Nave's Topical Index—AKJV

Exodus 9:12
And the LORD hardened the heart of Pharaoh, and he listened not to them; as the LORD had spoken to Moses.
Nave's Topical Index—AKJV