1 Chronicles 24:1-3

These were the divisions of the descendants of Aaron: The sons of Aaron were Nadab, Abihu, Eleazar and Ithamar. But Nadab and Abihu died before their father did, and they had no sons; so Eleazar and Ithamar served as the priests. With the help of Zadok a descendant of Eleazar and Ahimelek a descendant of Ithamar, David separated them into divisions for their appointed order of ministering.