Exodus 8:23

21Else, if you will not let my people go, behold, I will send swarms of flies on you, and on your servants, and on your people, and into your houses. The houses of the Egyptians shall be full of swarms of flies, and also the ground they are on.

22I 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 you may know that I am the LORD on the earth.

23I will put a division between my people and your people. This sign shall happen by tomorrow.”’”

24The LORD did so; and there came grievous swarms of flies into the house of Pharaoh, and into his servants’ houses. In all the land of Egypt the land was corrupted by reason of the swarms of flies.

25Pharaoh called for Moses and for Aaron, and said, “Go, sacrifice to your God in the land!”