Num 8:19

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 I have given the Levites as a gift to Aaron and his sons from among the Israelites, to perform the service for the Israelites at the Tent of Meeting and to make atonement on their behalf, so that no plague will come against the Israelites when they approach the sanctuary.”