Exodus 8:23

19 “This is the finger of God,” the magicians said to Pharaoh. But Pharaoh’s heart was hardened, and he would not listen to them, just as the LORD had said.

20 Then the LORD said to Moses, “Get up early in the morning, and when Pharaoh goes out to the water, stand before him and tell him that this is what the LORD says: ‘Let My people go, so that they may worship Me.

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.”

Read all of Exodus 8