Exodus 7:22-25

22And the scribes of Egypt did so with their sorceries; and Pharaoh's heart was stubborn, neither did he hearken to them, as Jehovah had said. 23And Pharaoh turned and went into his house, and took not this to heart either. 24And all the Egyptians dug round about the river for water to drink; for they could not drink of the water of the river. 25And seven days were fulfilled, after Jehovah had smitten the river.