Exodus 7:22-25

Then the magicians of Egypt did so with their enchantments; and Pharaoh’s heart grew hard, and he did not heed them, as the LORD had said. And Pharaoh turned and went into his house. Neither was his heart moved by this. So all the Egyptians dug all around the river for water to drink, because they could not drink the water of the river. And seven days passed after the LORD had struck the river.