Exodus 10:21-23

Then the Lord said to Moses: "Extend your hand into the sky. And let there be a darkness over the land of Egypt, so dense that they may be able to feel it." And Moses extended his hand into the sky. And there came a horrible darkness in the entire land of Egypt for three days. No one saw his brother, nor moved himself out of the place where he was. But wherever the sons of Israel were living, there was light.