Exodus 7:15-18

15Go to Pharaoh in the morning as he is coming out to the water, and stand ready to meet him by the bank of the Nile. Take the staff that was transformed into a serpent in your hand.

16You are to say to him: Adonai, God of the Hebrews, has sent me to you, saying, ‘Let My people go, so they may serve Me in the wilderness,’ and behold, you have not listened.

17This is what Adonai says: ‘By this you will know that I am Adonai. Behold, I will strike the waters that are in the river with the staff that is in my hand, and they will be turned to blood.

18The fish that are in the river will die, the river will become foul, and the Egyptians will hate to drink water from the Nile.”’