Then the Lord said to Moses, "Tell Aaron: Stretch out your staff and strike the dust of the earth, and it will become gnats throughout the land of Egypt."
References for Exodus 8:16
And they did this. Aaron stretched out his hand with his staff, and when he struck the dust of the earth, gnats were on the people and animals. All the dust of the earth became gnats throughout the land of Egypt.
The magicians tried to produce gnats using their occult practices, but they could not. The gnats remained on the people and animals.
"This is the finger of God," the magicians said to Pharaoh. But Pharaoh's heart hardened, and he would not listen to them, as the Lord had said.