Exodus 8:15-19

But when Pharaoh saw that there was relief, he hardened his heart and would not listen to Moses and Aaron, just as the LORD had said. Then the LORD said to Moses, “Tell Aaron, ‘Stretch out your staff and strike the dust of the ground,’ and throughout the land of Egypt the dust will become gnats.” They did this, and when Aaron stretched out his hand with the staff and struck the dust of the ground, gnats came on people and animals. All the dust throughout the land of Egypt became gnats. But when the magicians tried to produce gnats by their secret arts, they could not. Since the gnats were on people and animals everywhere, the magicians said to Pharaoh, “This is the finger of God.” But Pharaoh’s heart was hard and he would not listen, just as the LORD had said.