Exodus 7:22-25

And 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. And Pharaoh turned and went into his house, neither did he lay even this to heart. And all the Egyptians digged round about the river for water to drink; for they could not drink of the water of the river. And seven days were fulfilled, after that Jehovah had smitten the river.