Exodus 8:29

8:29 Moses said, “I am going to go out from you and pray to the Lord, and the swarms of flies will go away from Pharaoh, from his servants, and from his people tomorrow. Only do not let Pharaoh deal falsely again by not releasing the people to sacrifice to the Lord.”

Exodus 14:4

14:4 I will harden Pharaoh’s heart, and he will chase after them. I will gain honor because of Pharaoh and because of all his army, and the Egyptians will know that I am the Lord.” So this is what they did.