Exodus 7:22-25

22And the magicians of Egypt did in like manner with their enchantments: and Pharaoh’s heart was hardened, and he hearkened not unto them; as Jehovah had spoken. 23And Pharaoh turned and went into his house, neither did he lay even this to heart. 24And all the Egyptians digged 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 that Jehovah had smitten the river.