1 Chronicles 24:3, 31

With the help of Zadok, who was a descendant of Eleazar, and of Ahimelech, who was a descendant of Ithamar, David divided Aaron’s descendants into groups according to their various duties.

Like the descendants of Aaron, they were assigned to their duties by means of sacred lots, without regard to age or rank. Lots were drawn in the presence of King David, Zadok, Ahimelech, and the family leaders of the priests and the Levites.