Exodus 7:14-17 NIV

Then the LORD said to Moses, “Pharaoh’s heart is unyielding; he refuses to let the people go. Go to Pharaoh in the morning as he goes out to the river. Confront him on the bank of the Nile, and take in your hand the staff that was changed into a snake. Then say to him, ‘The LORD, the God of the Hebrews, has sent me to say to you: Let my people go, so that they may worship me in the wilderness. But until now you have not listened. This is what the LORD says: By this you will know that I am the LORD: With the staff that is in my hand I will strike the water of the Nile, and it will be changed into blood.