The Exodus page 4
Moses and Aaron went to Pharaoh and delivered that message. “The Lord God of Israel says, ‘Let My people go’.”
But Pharaoh demanded, “Who is the Lord, that I should obey His voice to let Israel go? I don’t know the Lord, neither will I let Israel go.”
So Moses returned to the Lord, asking, “Lord, why did you send me? I went to Pharaoh to speak in your name, but You have not delivered Your people.”
Then, the Lord explained to Moses that in Egypt He would soon reveal Himself as Jehovah, the Savior, and there also He would prove to the Egyptians that all people must worship the one true God, not His creations. Then the Lord said to Moses and Aaron, “Go before Pharaoh. Take your staff, and stretch out your hand on the waters of Egypt.”
Moses and Aaron did as the Lord commanded, and struck the river; and all the water in the river turned to blood. The fish died, and the river stank, and the Egyptians could not drink any of the water throughout all the land of Egypt.