But the LORD hardened Pharaoh's heart, and he did not let the children of Israel go.
Exodus 10:20
Surrounding context
10:19 And the LORD turned a very strong west wind, which took the locusts away and blew them into the Red Sea. There remained not one locust in all the territory of Egypt.
10:20 But the LORD hardened Pharaoh's heart, and he did not let the children of Israel go.
10:21 Then the LORD said to Moses, "Stretch out your hand toward heaven, that there may be darkness over the land of Egypt, darkness which may even be felt."