Exodus 7:22-25

But the magicians of Egypt did the same by their secret arts and enchantments; so Pharaoh’s heart was hardened, and he did not listen to Moses and Aaron, just as the LORD had said. Then Pharaoh turned and went into his house, and he did not take even this [divine sign] to heart. So all the Egyptians dug near the river for water to drink, because they could not drink the water of the Nile. Seven days passed after the LORD had struck the Nile.