I will harden his heart, that he shall not let the people go. Exodus 4:21
The LORD hardened Pharaoh's heart, so that he would not let the children of Israel go. Exodus 10:20
The LORD hardened Pharaoh's heart, so that he would not let the children of Israel go out of his land. Exodus 11:10
He also did it to show off his signs and wonders.
And the LORD said unto Moses, Go in unto Pharaoh: for I have hardened his heart, and the heart of his servants, that I might shew these my signs before him. Exodus 10:20
To let the Egyptians know that he is the Lord.
And to get him honor.
And I will harden Pharaoh's heart, that he shall follow after them; and I will be honoured upon Pharaoh, and upon all his host; that the Egyptians may know that I am the LORD. Exodus 14:4
I will harden the hearts of the Egyptians, and they shall follow them: and I will get me honour. Exodus 14:18