Exodus 10:20-22

Young's Literal Translation

²⁰ and Jehovah strengtheneth the heart of Pharaoh, and he hath not sent the sons of Israel away.

²¹ And Jehovah saith unto Moses, `Stretch out thy hand towards the heavens, and there is darkness over the land of Egypt, and the darkness is felt.'

²² And Moses stretcheth out his hand towards the heavens, and there is darkness -- thick darkness in all the land of Egypt three days;