Numbers 3:4
And Nadab and Abihu died before the LORD, when they offered strange fire before the LORD, in the wilderness of Sinai, and they had no children: and Eleazar and Ithamar ministered in the priest’s office in the sight of Aaron their father. And Nadab and Abihu died before the LORD, when they offered strange fire before the LORD, in the wilderness of Sinai, and they had no children: and Eleazar and Ithamar ministered in the priest’s office in the sight of Aaron their father.
Numbers 3:1-13 identifies Aaron as a Levite and places the priesthood within the tribe of Levi. That priesthood was vested in Levi for its appointed age. Levi itself, the tribe to which the priesthood belonged, belonged to God. The Aaronic priesthood was established by law to continue until it was fulfilled in its type by the priesthood of Christ, and during that dispensation no one outside the appointed tribe could minister in the priest's office.
Nadab and Abihu were slain for offering strange fire before the Lord in His tabernacle, and their bodies were then carried away from before the sanctuary. After their death Eleazar and Ithamar remained as the only legitimate priests, and the priesthood continued through their families. The chapter distinguishes the duties of Aaron's sons and assigns to the Kohathites the task of carrying the ark and the sacred vessels, with the ark to be borne on their shoulders and not transported on a cart, and with the sacred objects not to be touched by them.