Cross reference
Exodus 9:35
35 The heart of Pharaoh was hardened, and he didn't let the children of Israel go, just as Yahweh had spoken through Moses.
Exodus 4:21
21 Yahweh said to Moses, "When you go back into Egypt, see that you do before Pharaoh all the wonders which I have put in your hand, but I will harden his heart and he will not let the people go.