Exodus 7:15

¹³  And he hardened Pharaoh's heart, that he hearkened not unto them; as the LORD had said.

¹⁴  And the LORD said unto Moses, Pharaoh's heart is hardened, he refuseth to let the people go.

¹⁵  Get thee unto Pharaoh in the morning; lo, he goeth out unto the water; and thou shalt stand by the river's brink against he come; and the rod which was turned to a serpent shalt thou take in thine hand.

¹⁶  And thou shalt say unto him, The LORD 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 wouldest not hear.

¹⁷  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.