Numbers 4:21

וְזֹאת עֲשׂוּ לָהֶם וְחָיוּ וְלֹא יָמֻתוּ בְּגִשְׁתָּם אֶת־קֹדֶשׁ הַקֳּדָשִׁים אַהֲרֹן וּבָנָיו יָבֹאוּ וְשָׂמוּ אוֹתָם אִישׁ אִישׁ עַל־עֲבֹדָתוֹ וְאֶל־מַשָּׂאֽוֹ

but thus do unto them, that they may live, and not die, when they approach unto the most holy things: Aaron and his sons shall go in, and appoint them every one to his service and to his burden;

וְלֹא־יָבֹאוּ לִרְאוֹת כְּבַלַּע אֶת־הַקֹּדֶשׁ וָמֵֽתוּ

but they shall not go in to see the holy things as they are being covered, lest they die.’

נָשֹׂא אֶת־רֹאשׁ בְּנֵי גֵרְשׁוֹן גַּם־הֵם לְבֵית אֲבֹתָם לְמִשְׁפְּחֹתָֽם

’Take the sum of the sons of Gershon also, by their fathers’houses, by their families;

מִבֶּן שְׁלֹשִׁים שָׁנָה וָמַעְלָה עַד בֶּן־חֲמִשִּׁים שָׁנָה תִּפְקֹד אוֹתָם כָּל־הַבָּא לִצְבֹא צָבָא לַעֲבֹד עֲבֹדָה בְּאֹהֶל מוֹעֵֽד

from thirty years old and upward until fifty years old shalt thou number them: all that enter in to wait upon the service, to do service in the tent of meeting.