Cross reference
Exodus 9:35
35 And the heart of Pharaoh was hardened, and he did not let the children of Israel go, as Jehovah had spoken by Moses.
Exodus 4:21
21 And Jehovah said unto Moses, When thou goest back into Egypt, see that thou do before Pharaoh all the wonders which I have put in thy hand: but I will harden his heart and he will not let the people go.