Exodus 8:20-24:
21"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.
22"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.
23"I will [a]put a division between My people and your people. Tomorrow this sign will occur."
24Then the LORD did so. And there came great swarms of flies into the house of Pharaoh..."