Numbers 28:2

28:2 “Command the Israelites: ‘With regard to my offering, be sure to offer my food for my offering made by fire, as a pleasing aroma to me at its appointed time.’

Numbers 18:7

18:7 But you and your sons with you are responsible for your priestly duties, for everything at the altar and within the curtain. And you must serve. I give you the priesthood as a gift for service; but the unauthorized person who approaches must be put to death.”