Exodus 8:20-22

20And the Lord said to Moses, “Rise early in the morning and stand before Pharaoh as he comes out to the water. Then say to him, ‘Thus says the Lord: “Let My people go, that they may serve Me.

21Or else, if you will not let My people go, behold, I will send swarms of flies on you and your servants, on your people and into your houses. The houses of the Egyptians shall be full of swarms of flies, and also the ground on which they stand.

22And in that day I will set apart the land of Goshen, in which My people dwell, that no swarms of flies shall be there, in order that you may know that I am the Lord in the midst of the land.