Exodus 8:25

21 But if you will not let My people go, I will send swarms of flies upon you and your officials and your people and your houses. The houses of the Egyptians and even the ground where they stand will be full of flies.

22 But on that day I will give special treatment to the land of Goshen, where My people live; no swarms of flies will be found there. In this way you will know that I, the LORD, am in the land.

23 I will make a distinction between My people and your people. This sign will take place tomorrow.’”

24 And the LORD did so. Thick swarms of flies poured into Pharaoh’s palace and into the houses of his officials. Throughout Egypt the land was ruined by swarms of flies.

25 Then Pharaoh summoned Moses and Aaron and said, “Go, sacrifice to your God within this land.”

26 But Moses replied, “It would not be right to do that, because the sacrifices we offer to the LORD our God would be detestable to the Egyptians. If we offer sacrifices that are detestable before the Egyptians, will they not stone us?

27 We must make a three-day journey into the wilderness and sacrifice to the LORD our God as He commands us.”

28 Pharaoh answered, “I will let you go and sacrifice to the LORD your God in the wilderness, but you must not go very far. Now pray for me.”

29 “As soon as I leave you,” Moses said, “I will pray to the LORD, so that tomorrow the swarms of flies will depart from Pharaoh and his officials and his people. But Pharaoh must not act deceitfully again by refusing to let the people go and sacrifice to the LORD.”

Read all of Exodus 8