Exodus 7:22-25

22But the Egyptian magicians did the same things by their secret arts, and Pharaoh’s heart became hard; he would not listen to Moses and Aaron, just as the Lord had said.

23Instead, he turned and went into his palace, and did not take even this to heart.

24And all the Egyptians dug along the Nile to get drinking water, because they could not drink the water of the river.

25Seven days passed after the Lord struck the Nile.