וְהִבְדַּלְתָּ֙ אֶת־הַלְוִיִּ֔ם מִתּ֖וֹךְ בְּנֵ֣י יִשְׂרָאֵ֑ל וְהָ֥יוּ לִ֖י הַלְוִיִּֽם

Thus you shall set the Levites apart from the Israelites, and the Levites shall be Mine. 

וְאַֽחֲרֵי־כֵן֙ יָבֹ֣אוּ הַלְוִיִּ֔ם לַעֲבֹ֖ד אֶת־אֹ֣הֶל מוֹעֵ֑ד וְטִֽהַרְתָּ֣ אֹתָ֔ם וְהֵנַפְתָּ֥ אֹתָ֖ם תְּנוּפָֽה

Thereafter the Levites shall be qualified for the service of the Tent of Meeting, once you have cleansed them and designated them as an elevation offering. 

כִּי֩ נְתֻנִ֨ים נְתֻנִ֥ים הֵ֙מָּה֙ לִ֔י מִתּ֖וֹךְ בְּנֵ֣י יִשְׂרָאֵ֑ל תַּחַת֩ פִּטְרַ֨ת כָּל־רֶ֜חֶם בְּכ֥וֹר כֹּל֙ מִבְּנֵ֣י יִשְׂרָאֵ֔ל לָקַ֥חְתִּי אֹתָ֖ם לִֽי

For they are formally assigned to Me from among the Israelites: I have taken them for Myself in place of all the first issue of the womb, of all the first-born of the Israelites. 

כִּ֣י לִ֤י כָל־בְּכוֹר֙ בִּבְנֵ֣י יִשְׂרָאֵ֔ל בָּאָדָ֖ם וּבַבְּהֵמָ֑ה בְּי֗וֹם הַכֹּתִ֤י כָל־בְּכוֹר֙ בְּאֶ֣רֶץ מִצְרַ֔יִם הִקְדַּ֥שְׁתִּי אֹתָ֖ם לִֽי

For every first-born among the Israelites, man as well as beast, is Mine; I consecrated them to Myself at the time that I smote every first-born in the land of Egypt. 

וָאֶקַּח֙ אֶת־הַלְוִיִּ֔ם תַּ֥חַת כָּל־בְּכ֖וֹר בִּבְנֵ֥י יִשְׂרָאֵֽל

Now I take the Levites instead of every first-born of the Israelites; 

וָאֶתְּנָ֨ה אֶת־הַלְוִיִּ֜ם נְתֻנִ֣ים ׀ לְאַהֲרֹ֣ן וּלְבָנָ֗יו מִתּוֹךְ֮ בְּנֵ֣י יִשְׂרָאֵל֒ לַעֲבֹ֞ד אֶת־עֲבֹדַ֤ת בְּנֵֽי־יִשְׂרָאֵל֙ בְּאֹ֣הֶל מוֹעֵ֔ד וּלְכַפֵּ֖ר עַל־בְּנֵ֣י יִשְׂרָאֵ֑ל וְלֹ֨א יִהְיֶ֜ה בִּבְנֵ֤י יִשְׂרָאֵל֙ נֶ֔גֶף בְּגֶ֥שֶׁת בְּנֵֽי־יִשְׂרָאֵ֖ל אֶל־הַקֹּֽדֶשׁ

and from among the Israelites I formally assign the Levites to Aaron and his sons, to perform the service for the Israelites in the Tent of Meeting and to make expiation for the Israelites, so that no plague may afflict the Israelites for coming too near the sanctuary. 

Numbers 8:14-19