במדבר ג׳:י״א-י״ג

ואני הנה לקחתי. וַאֲנִי מֵהֵיכָן זָכִיתִי בָהֶן מתוך בני ישראל שֶׁיִּהְיוּ יִשְׂרָאֵל שׂוֹכְרִין אוֹתָן לְשֵׁרוּת שֶׁלִּי? עַ"יְ הַבְּכוֹרוֹת זָכִיתִי בָהֶם וּלְקַחְתִּים תְּמוּרָתָם. לְפִי שֶׁהָיְתָה הָעֲבוֹדָה בַּבְּכוֹרוֹת, וּכְשֶׁחָטְאוּ בָעֵגֶל נִפְסְלוּ, וְהַלְוִיִּם שֶׁלֹּא עָבְדוּ עֲ"זָ נִבְחֲרוּ תַחְתֵּיהֶם (עי' זבחים קי"ב):

Behold, I have taken [the Levites]. What is My right to them? From among Bnei Yisroel That the Israelites must hire them to serve Me? Through the [Israelites’] first born, I gained my right to them, having taken them (the Levites) in their place. Because the sacred service was [originally performed] by the firstborn, but when they sinned with the golden calf, they were disqualified. The Levites, who did not indulge in idolatry, were chosen instead.