Numbers 18:7

And you and your sons with you shall guard your priesthood for all that concerns the altar and that is within the veil ; and you shall serve . I give your priesthood as a gift , and any outsider who comes near shall be put to death .

But only you and your sons may serve as priests in connection with everything at the altar and inside the curtain. I am giving you the service of the priesthood as a gift. Anyone else who comes near the sanctuary is to be put to death.”

Therefore thou and thy sons with thee shall keep your priest's office for every thing of the altar, and within the vail; and ye shall serve : I have given your priest's office unto you as a service of gift : and the stranger that cometh nigh shall be put to death.