Exodus 8:21-24

For if you do 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; and the houses of the Egyptians will be full of swarms of flies, and also the ground on which they dwell. But on that day I will set apart the land of Goshen, where My people are living, so that no swarms of flies will be there, in order that you may know that I, the LORD, am in the midst of the land. I will put a division between My people and your people. Tomorrow this sign will occur.” ’ ” Then the LORD did so. And there came great swarms of flies into the house of Pharaoh and the houses of his servants and the land was laid waste because of the swarms of flies in all the land of Egypt.