Numbers 3:1-13

Now these are the records of the generations of Aaron and Moses in the day when the LORD spoke [by special revelation] with Moses on Mount Sinai. These are the names of the sons of Aaron: Nadab the firstborn, Abihu, Eleazar, and Ithamar. These are the names of the sons of Aaron, the priests who were anointed, whom Aaron consecrated and ordained to minister in the priest’s office. But Nadab and Abihu died in the presence of the LORD when they offered strange (unholy, unacceptable, inappropriate) fire before the LORD in the Wilderness of Sinai; and they had no sons. So Eleazar and Ithamar served as priests in the presence and under the supervision of Aaron their father. [Lev 10:1-4] Then the LORD spoke to Moses, saying, “Bring the tribe of Levi near and present them before Aaron the priest, so that they may serve him. They shall carry out the duties for him and for the whole congregation before the Tent of Meeting, doing the service of the tabernacle. They shall also take care of all the furnishings and utensils of the Tent of Meeting, and [attend to] the duties of the Israelites, doing the service of the tabernacle. You shall give the Levites to Aaron and to his sons [as servants and helpers]; they are to be given wholly to him from among the Israelites. So you shall appoint Aaron and his sons, and they shall observe and attend to their priesthood; but the layman (non-Levite) who approaches [the holy things] shall be put to death.” Again the LORD spoke to Moses, saying, “Behold, I have taken the Levites from among the sons of Israel instead of and as a substitute for every firstborn, that is, the first that opens the womb among the Israelites. So the Levites shall be Mine, for all the firstborn are Mine. On the day that I killed all the firstborn in the land of Egypt, I consecrated for Myself all the firstborn in Israel, both man and animal. They shall be mine; I am the LORD.”