Cross reference
Exodus 9:35
35 And the heart of Pharaoh was hard, and he did not let the people go, as the Lord had said by the mouth of Moses.
Exodus 4:21
21 And the Lord said to Moses, When you go back to Egypt, see that you do before Pharaoh all the wonders which I have given you power to do: but I will make his heart hard and he will not let the people go.