Numbers 3:12

ואני הנה לקחתי AND I, BEHOLD I HAVE TAKEN [THE LEVITES FROM AMONG THE CHILDREN OF ISRAEL] — This means: And I — whence do “I” claim the right on them from among the children of Israel, that the Israelites should have to hire them (cf. Rashi on v. 8) for My service? It is through the firstborn — to whom I surely have a title — that I have a claim on them (the Levites) and so I take them in their (the firstborns’) stead. For originally the service (the priestly functions) was performed by the firstborn, but when they (the Israelites and among them their firstborn too) sinned by worshipping the golden calf they became disqualified, and the Levites who had not worshipped the idol were chosen in their stead (cf. Zevachim 112b).

And I, behold, I have taken the Levites from among the children of Yisra᾽el instead of all the firstborn that opens the womb among the children of Yisra᾽el: therefore the Levites shall be mine;

וַאֲנִ֞י הִנֵּ֧ה לָקַ֣חְתִּי אֶת־הַלְוִיִּ֗ם מִתּוֹךְ֙ בְּנֵ֣י יִשְׂרָאֵ֔ל תַּ֧חַת כׇּל־בְּכ֛וֹר פֶּ֥טֶר רֶ֖חֶם מִבְּנֵ֣י יִשְׂרָאֵ֑ל וְהָ֥יוּ לִ֖י הַלְוִיִּֽם׃