Num 8:26

וְשֵׁרֵ֨ת אֶת־אֶחָ֜יו בְּאֹ֤הֶל מוֹעֵד֙ לִשְׁמֹ֣ר מִשְׁמֶ֔רֶת וַעֲבֹדָ֖ה לֹ֣א יַעֲבֹ֑ד כָּ֛כָה תַּעֲשֶׂ֥ה לַלְוִיִּ֖ם בְּמִשְׁמְרֹתָֽם׃ פ

After that, they may assist their brothers in fulfilling their duties at the Tent of Meeting, but they themselves are not to do the work. This is how you are to assign responsibilities to the Levites.”