Exodus 8:21-24

Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are. And I will set apart in that day the land of Goshen, in which my people dwell, that no swarms of flies shall be there; to the end thou mayest know that I am Jehovah in the midst of the earth. And I will put a division between my people and thy people: by to-morrow shall this sign be. And Jehovah did so; and there came grievous swarms of flies into the house of Pharaoh, and into his servants’ houses: and in all the land of Egypt the land was corrupted by reason of the swarms of flies.