Numbers 3:2-4

The names of the sons of Aaron: Nadab the firstborn, Abihu, Eleazar, and Ithamar—anointed priests ordained to serve as priests. But Nadab and Abihu fell dead in the presence of GOD when they offered unauthorized sacrifice to him in the Wilderness of Sinai. They left no sons, and so only Eleazar and Ithamar served as priests during the lifetime of their father, Aaron.