TSK
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"Behold, I have taken the Levites from among the children of Israel instead of all the firstborn who open the womb among the children of Israel; and the Levites shall be mine:
from thirty years old and upward even until fifty years old, all who enter into the service, to do the work in the Tent of Meeting.
You shall set the Levites before Aaron, and before his sons, and offer them as a wave offering to Yahweh.
Moreover, David and the captains of the army set apart for the service certain of the sons of Asaph, and of Heman, and of Jeduthun, who should prophesy with harps, with stringed instruments, and with cymbals: and the number of those who did the work according to their service was: