Midrasch zu Schemot 7:17

Get thee unto Pharaoh in the morning; lo, he goeth out unto the water (Exod. 7:14). Thus the Lord said: Through this (water) you shall cause him to know that I am the Lord. I will smite the water of the Nile with the staff that is in your hand, and it shall turn to blood. Why were the waters turned to blood first? Because Pharaoh and the Egyptians worshipped the Nile, and the Holy One, blessed be He, said: Smite his god first and then him, as in the popular aphorism, “Strike the gods and the priests will be frightened.” And the Lord said unto Moses: “Say unto Aaron: Stretch out thy hand, and smite” (Exod. 7:19). Smite the water of the Nile, the roots of the plants in the rivers, and indeed, every source of their water, even that which is in their ladles. And blood appeared in the wood and stone, and even the spittle of the Egyptians turned to blood.

(Exod. 7:14, cont.:) HE REFUSES TO LET <THE PEOPLE> GO (shillah). The Holy One said: You are refusing to let them go. By your life, you yourself will take each and every one of them by the hand and send them away (shillah). The Holy One said to Moses: Go and bring the plague of blood upon them. Then you shall say unto them (in Exod. 7:17): BY THIS YOU SHALL KNOW THAT I AM THE LORD…. Why did he bring upon them the plague of blood first?⁷⁷Tanh., Exod. 2:13 Exod. R. 9:9. Because Pharaoh and the Egyptians were worshiping the Nile. The Holy One had said to Moses: Go and smite their gods in their presence. A common⁷⁸Gk. idotes. (Aramaic) proverb says: Blot out the god and the idol-priests are confounded {i.e., smite the idol and the idol-priests are confounded}. Ergo (in Exod. 7:17, cont.:) SEE, I AM SMITING < THE WATER WHICH IS IN THE NILE WITH THE ROD THAT IS IN MY HAND>…. See what is written (in vs. 19): <TAKE YOUR ROD AND STRETCH OUT YOUR HAND OVER THE WATERS OF EGYPT: > OVER ITS RIVERS, <OVER ITS CANALS, OVER ITS PONDS, AND OVER EVERY BODY OF ITS WATER SO THAT THEY BECOME BLOOD>.⁷⁹I.e., not simply OVER THE WATERS OF EGYPT. What is the meaning of OVER ITS RIVERS? In every place where they were, the water became blood. What is the meaning of OVER EVERY BODY OF ITS WATER? Even what was in the cup⁸⁰Gk.: kothon. became blood, and even what an Egyptian was spitting out of his mouth became blood.⁸¹Exod. R.9:10. Thus it is stated (in ibid., cont.): AND THERE SHALL BE BLOOD IN THE WHOLE LAND OF EGYPT. R. Abbin the Levite said: Israel became rich from the plague of blood.⁸²M. Ps. 78:10. How? When an Egyptian and an Israelite were in the same house, the tank was full of water; but when the Egyptian went to fill a cup from it, it became blood. So when the Israelite went and drank water from it, the Egyptian would say: Give me a little of the water in your hand; but when he gave it to him, it turned out to be blood in the Egyptian's hand. Then he would say to him: Come, you and I, and let us drink from the <same> bowl. But the Israelite was < still > drinking water; and the Egyptian, blood. When, however, an Egyptian took water from an Israelite for a price, he would drink water. For that reason Israel became rich from the plague of blood. And how long did the plague stay with them? R. Judah and R. Nehemiah differed.⁸³Exod. R. 9:12. The one said: <The Holy One> warned them for twenty-four days, while the < actual > plague functioned against them for seven days. But the other said: He warned them for seven days, while the < actual > plague functioned against them for twenty-four days.