Exodus 7:20-22

20So Moses and Aaron did as Adonai commanded. He lifted up the staff and struck the waters that were in the river in the sight of Pharaoh and his servants, and all the waters of the Nile turned to blood.

21When the fish that were in the river died, the river became so foul that the Egyptians could not drink water from the river. The blood was throughout all the land of Egypt.

22But the magicians of Egypt did the same with their secret arts. So Pharaoh’s heart was hardened, and he did not listen to them—just as Adonai had said.