Then the LORD said to Moses, “Pharaoh's heart is hardened; he refuses to let the people go. Go to Pharaoh in the morning, as he is going out to the water. Stand on the bank of the Nile to meet him, and take in your hand the staff that turned into a serpent. And you shall say to him, ‘The LORD, the God of the Hebrews, sent me to you, saying, “Let my people go, that they may serve me in the wilderness.” But so far, you have not obeyed. Thus says the LORD, “By this you shall know that I am the LORD: behold, with the staff that is in my hand I will strike the water that is in the Nile, and it shall turn into blood. The fish in the Nile shall die, and the Nile will stink, and the Egyptians will grow weary of drinking water from the Nile.”’”

Then the LORD said to Moses, “Stretch out your hand toward heaven, that there may be darkness over the land of Egypt, a darkness to be felt.” So Moses stretched out his hand toward heaven, and there was pitch darkness in all the land of Egypt three days. They did not see one another, nor did anyone rise from his place for three days, but all the people of Israel had light where they lived. Then Pharaoh called Moses and said, “Go, serve the LORD; your little ones also may go with you; only let your flocks and your herds remain behind.” But Moses said, “You must also let us have sacrifices and burnt offerings, that we may sacrifice to the LORD our God. Our livestock also must go with us; not a hoof shall be left behind, for we must take of them to serve the LORD our God, and we do not know with what we must serve the LORD until we arrive there.” But the LORD hardened Pharaoh's heart, and he would not let them go.

(Exodus 7:14-18, 10:21-27 ESV)