“You shall say to him: The Lord, God of the Hebrews, has sent me to you, saying: Let My people go, and they will serve Me in the wilderness; and, behold, you have not heeded until now. So said the Lord: With this you will know that I am the Lord. Behold, I will strike with the staff that is in my hand on the water that is in the Nile, and it will be turned into blood” (Exodus 7:16–17). “You shall say to him: The Lord, God of the Hebrews.… So said the Lord: With this you will know” – this is what is written: “Behold, God is exalted in His power, who is a teacher like Him?” (Job 36:22).
It is the way of the world that when a human being seeks to bring harm to his enemy, he brings it upon him suddenly so he will not sense it [is coming]; but the Holy One blessed be He forewarned Pharaoh for each and every plague so he would repent; this is what is written: “With this you will know that I am the Lord”; “Behold, I will strike all your borders with frogs (Exodus 7:27); “send and gather” (Exodus 9:19).13This is the forewarning that preceded the plague of hail.
“On the water that is in the Nile, and it will be turned into blood” – why was the water afflicted first with blood? It is because Pharaoh and the Egyptians worshipped the Nile. The Holy One blessed be He said: I will smite the god first and then his people. This is a folk parable: Smite the gods and the priests will be afraid.
Likewise it says: “The Lord will reckon with the host of the high heavens on high,” and after that, [“the kings of the earth upon the earth”] (Isaiah 24:21).