Exodus 10:25 (KJV)

But the LORD hardened Pharaoh's heart, so that he would not let the children of Israel go.

And the LORD said unto Moses, Stretch out thine hand toward heaven, that there may be darkness over the land of Egypt, even darkness which may be felt.

And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt three days:

They saw not one another, neither rose any from his place for three days: but all the children of Israel had light in their dwellings.

And Pharaoh called unto Moses, and said, Go ye, serve the LORD; only let your flocks and your herds be stayed: let your little ones also go with you.

Our cattle also shall go with us; there shall not an hoof be left behind; for thereof must we take to serve the LORD our God; and we know not with what we must serve the LORD, until we come thither.

But the LORD hardened Pharaoh's heart, and he would not let them go.

And Pharaoh said unto him, Get thee from me, take heed to thyself, see my face no more; for in that day thou seest my face thou shalt die.

And Moses said, Thou hast spoken well, I will see thy face again no more.