Exodus 7:20-21

And Moses and Aaron did so, just as the LORD commanded. So he lifted up the rod and struck the waters that were in the river, in the sight of Pharaoh and in the sight of his servants. And all the waters that were in the river were turned to blood. The fish that were in the river died, the river stank, and the Egyptians could not drink the water of the river. So there was blood throughout all the land of Egypt.