Exodus 8:20-24

20Then 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. 21But if you will not let My people go, I will send swarms of flies1 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.

22But 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. 23I will make a distinction2 between My people and your people. This sign will take place tomorrow.’”

24And 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.