Numbers 18:1, 7

Then the LORD said to Aaron: “You, your sons, and your relatives from the tribe of Levi will be held responsible for any offenses related to the sanctuary. But you and your sons alone will be held responsible for violations connected with the priesthood.

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.”