Exodus 8:21-23

Contemporary English Version

²¹ If you don't, he will send swarms of flies to attack you, your officials, and every citizen of your country. Your houses will be full of flies, and the ground will crawl with them.

^22-23 “The Lord's people in Goshen won't be bothered by flies, but your people in the rest of the country will be tormented by them. That's how you will know that the Lord is here in Egypt. This miracle will happen tomorrow.”