Exodus 7:22-25

But again the magicians of Egypt used their magic, and they, too, turned water into blood. So Pharaoh’s heart remained hard. He refused to listen to Moses and Aaron, just as the LORD had predicted. Pharaoh returned to his palace and put the whole thing out of his mind. Then all the Egyptians dug along the riverbank to find drinking water, for they couldn’t drink the water from the Nile. Seven days passed from the time the LORD struck the Nile.