Numbers 3:1-4 ESV
These are the generations of Aaron and Moses at the time when the LORD spoke with Moses on Mount Sinai. These are the names of the sons of Aaron: Nadab the firstborn, and Abihu, Eleazar, and Ithamar. These are the names of the sons of Aaron, the anointed priests, whom he ordained to serve as priests. But Nadab and Abihu died before the LORD when they offered unauthorized fire before the LORD in the wilderness of Sinai, and they had no children. So Eleazar and Ithamar served as priests in the lifetime of Aaron their father.
ESV: English Standard Version 2016