1 Chronicles 24:1-3

Now the divisions of the descendants of Aaron were these: the sons of Aaron were Nadab, Abihu, Eleazar, and Ithamar.

But Nadab and Abihu died before their father and had no sons. So Eleazar and Ithamar served as priests.

David, with Zadok of the sons of Eleazar and Ahimelech of the sons of Ithamar, divided them according to their offices for their ministry.