Exodus 8:20-24

20The Lord said to Moses, “Get up early in the morning and present yourself to Pharaoh when you see him going out to the water. Tell him: 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, then I will send swarms of flies against you, your officials, your people, and your houses. The Egyptians’ houses will swarm with flies, and so will the land where they live.

22But on that day I will give special treatment to the land of Goshen, where my people are living; no flies will be there. This way you will know that I, the Lord, am in the land.

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

24And the Lord did this. Thick swarms of flies went into Pharaoh’s palace and his officials’ houses. Throughout Egypt the land was ruined because of the swarms of flies.