Exodus 7:20
"And Moses and Aaron did so, as the LORD commanded; and he lifted up the rod, and smote 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."
Cross References (ranked by relevance)
Psalm 78:44
And had turned their rivers into blood; and their floods, that they could not drink.
Psalm 105:29
He turned their waters into blood, and slew their fish.
Revelation 8:8
And the second angel sounded, and as it were a great mountain burning with fire was cast into the sea: and the third part of the sea became blood;