Exodus 10:21-23

Then ADONAI said to Moses, “Stretch out your hand toward heaven, and there will be darkness over the land of Egypt—a darkness that may be felt.” So Moses stretched out his hand toward heaven, and there was a thick darkness in all the land of Egypt for three days. They could not see one another, nor could anyone rise from his place for three days. Yet all Bnei-Yisrael had light within their dwellings.