Exodus 7:14-18

Then the LORD said to Moses, “Pharaoh’s heart is hard; he refuses to let the people go. Go to Pharaoh in the morning as he is going out to the water, and wait for him on the bank of the Nile; and you shall take in your hand the staff that was turned into a serpent. You shall say to him, ‘The LORD, the God of the Hebrews, has sent me to you, saying, “Let My people go, so that they may serve Me in the wilderness. But behold, you have not listened until now.” Thus says the LORD, “By this you shall know and recognize and acknowledge that I am the LORD: look, with the staff in my hand I will strike the water in the Nile, and it shall be turned to blood. The fish in the Nile will die, and the Nile will become foul, and the Egyptians will not be able to drink water from the Nile.” ’ ”