Numbers 3 - 4

1 Now these are the records of Aaron and Moses when the Lord spoke with Moses on Mount Sinai. 2 And these are the names of the sons of Aaron: Nadab, the firstborn, and Abihu, Eleazar, and Ithamar. 3 These are the names of the sons of Aaron, the anointed priests, whom he consecrated to minister as priests. 4 Nadab and Abihu had died before the Lord when they offered profane fire before the Lord in the Wilderness of Sinai; and they had no children. So Eleazar and Ithamar ministered as priests in the presence of Aaron their father.
5 And the Lord spoke to Moses, saying: 6 “Bring the tribe of Levi near, and present them before Aaron the priest, that they may serve him. 7 And they shall attend to his needs and the needs of the whole congregation before the tabernacle of meeting, to do the work of the tabernacle. 8 Also they shall attend to all the furnishings of the tabernacle of meeting, and to the needs of the children of Israel, to do the work of the tabernacle. 9 And you shall give the Levites to Aaron and his sons; they are given entirely to him from among the children of Israel. 10 So you shall appoint Aaron and his sons, and they shall attend to their priesthood; but the outsider who comes near shall be put to death.”
11 Then the Lord spoke to Moses, saying: 12 “Now behold, I Myself have taken the Levites from among the children of Israel instead of every firstborn who opens the womb among the children of Israel. Therefore the Levites shall be Mine, 13 because all the firstborn are Mine. On the day that I struck all the firstborn in the land of Egypt, I sanctified to Myself all the firstborn in Israel, both man and beast. They shall be Mine: I am the Lord ...”
— NUMBERS 3 - 4