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.
Exodus 7:17
Surrounding context
7:16 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!
7:17 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.
7:18 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.""'