Exodus 10:21-23

And the Lord said to Moses, Stretch out your hand toward the heavens, that there may be darkness over the land of Egypt, a darkness which may be felt. So Moses stretched out his hand toward the sky, and for three days a thick darkness was all over the land of Egypt. The Egyptians could not see one another, nor did anyone rise from his place for three days; but all the Israelites had natural light in their dwellings.