Exodus 7:17 in the King James Version
Thus saith the LORD, In this thou shalt know that I am the LORD: behold, I will smite with the rod that is in mine hand upon the waters which are in the river, and they shall be turned to blood.
Context
16 You shall tell him, ‘The LORD, the God of the Hebrews, has sent me to you, saying, “Let my people go, that they may serve me in the wilderness. Behold, until now you haven’t listened.”
17 The LORD says, “In this you shall know that I am the LORD. Behold: I will strike with the rod that is in my hand on the waters which are in the river, and they shall be turned to blood.
18 The fish that are in the river will die and the river will become foul. The Egyptians will loathe to drink water from the river.”’”