Midrash su Ezechiele 20:2

"And take the Leviim for me" (Numbers 3:41), that they will be taken for My Name. "I am the Eternal" (ibid.), I am trusted to pay a good reward that they are taken for my name. "In place of every firstborn among the Children of Israel" (ibid.), since the firstborns are Mine, as it says (Numbers 8:17) "For Mine is every firstborn [...] on the day that I smote every firstborn in the land of Egypt... [I consecrated them to Myself]", the Holy Blessed One says, for the sake of their obligation of Israel I changed the order of the world. With which side did I write in the Torah that a donkey may be redeemed with a lamb, as it says (Exodus 13:13) "The firstborn [lit. "opening"] of every donkey you shall redeem with a sheep"? I didn't do this; rather, I redeemed a sheep with a donkey. The Egyptians are called a donkey, as it says (Ezekiel 20:2) "That the flesh of donkeys is their flesh" and Israel is called a sheep, as it says (Jeremiah 50:17) "Israel is a scattered sheep". And I murdered their firstborn, and I sanctified the firstborn of Israel, as it says "For Mine is every firstborn among the children of Israel ... on the day that I struck all the firstborn ... [of Egypt]" (Numbers 3:41). And the animals of the Leviim in place of all the firstborns amongst the animals of the Children of Israel, that they you brought out to kill the place the firstborn of the animals of Egypt, and saved the animals of Israel, and therefore sanctified to His Name the firstborn of the animals of Israel, and since he says "For Mine is every firstborn among the children of Israel ... on the day that I struck all the firstborn ... [of Egypt]" (Numbers 3:41).