Exodus 7:16-17

7:16 Tell him, ‘The Lord, the God of the Hebrews, has sent me to you to say, “Release my people, that they may serve me in the desert!” But until now you have not listened. 7:17 Thus says the Lord: “By this you will know that I am the Lord: I am going to strike the water of the Nile with the staff that is in my hand, and it will be turned into blood.