Numbers 18:4, 7

The Levites must join you in fulfilling their responsibilities for the care and maintenance of the Tabernacle, but no unauthorized person may assist you.

But you and your sons, the priests, must personally handle all the priestly rituals associated with the altar and with everything behind the inner curtain. I am giving you the priesthood as your special privilege of service. Any unauthorized person who comes too near the sanctuary will be put to death.”