Exodus 7:14-18

So the LORD said to Moses: “Pharaoh’s heart is hard; he refuses to let the people go. Go to Pharaoh in the morning, when he goes out to the water, and you shall stand by the river’s bank to meet him; and the rod which was turned to a serpent you shall take in your hand. And you shall say to him, ‘The LORD God of the Hebrews has sent me to you, saying, “Let My people go, that they may serve Me in the wilderness”; but indeed, until now you would not hear! Thus says the LORD: “By this you shall know that I am the LORD. Behold, I will strike the waters which are in the river with the rod that is in my hand, and they shall be turned to blood. And the fish that are in the river shall die, the river shall stink, and the Egyptians will loathe to drink the water of the river.” ’ ”