Exodus 7:14-18

And Jehovah said unto Moses, Pharaoh’s heart is stubborn, he refuseth to let the people go. Get thee unto Pharaoh in the morning; lo, he goeth out unto the water; and thou shalt stand by the river’s brink to meet him; and the rod which was turned to a serpent shalt thou take in thy hand. And thou shalt say unto him, Jehovah, the God of the Hebrews, hath sent me unto thee, saying, Let my people go, that they may serve me in the wilderness: and, behold, hitherto thou hast not hearkened. Thus saith Jehovah, In this thou shalt know that I am Jehovah: behold, I will smite with the rod that is in my hand upon the waters which are in the river, and they shall be turned to blood. And the fish that are in the river shall die, and the river shall become foul; and the Egyptians shall loathe to drink water from the river.