Exodus 8:16-19

16Then Adonai said to Moses, “Rise up early in the morning and stand before Pharaoh. As he comes to the water say to him, This is what Adonai says: Let My people go, that they may serve Me.

17Or else, if you do not let My people go, I will send the swarm of flies on you and on your servants, on your people and into your houses. The houses of the Egyptians will be full of the swarm of flies including the ground that they stand on.

18“But on that day I will set apart the land of Goshen, where My people are dwelling—except no swarm of flies will be there—so that you may know that I, Adonai, am in the midst of the earth.

19I will make a distinction between My people and your people. By tomorrow this sign will happen.”