Exodus 7:22-25

But the magicians of Egypt did the same by their secret arts. So Pharaoh’s heart remained hardened, and he would not listen to them, as the LORD had said. Pharaoh turned and went into his house, and he did not take even this to heart. And all the Egyptians dug along the Nile for water to drink, for they could not drink the water of the Nile. Seven full days passed after the LORD had struck the Nile.