וְלִבְנֵ֥י אַהֲרֹ֖ן מַחְלְקוֹתָ֑ם בְּנֵ֣י אַהֲרֹ֔ן נָדָב֙ וַאֲבִיה֔וּא אֶלְעָזָ֖ר וְאִיתָמָֽר׃
And the courses of the sons of Aaron were these. The sons of Aaron: Nadab and Abihu, Eleazar and Ithamar.
Rashi
And the priests...on their watches as David had divided them into twenty-four watches, for each watch to serve its week, and the Levites likewise on their twenty-four watches
Rashi
in the watches as we find (in I Chronicles from ch. 24 on) that the Levites were divided into twenty- four watches.