Charles Ellicott Commentary Ezra 6:18

"And they set the priests in their divisions, and the Levites in their courses, for the service of God, which is at Jerusalem; as it is written in the book of Moses." — Ezra 6:18 (ASV)

In the book of Moses. —The general arrangements only were given in the Pentateuch. The “courses” were of David’s time; and their restoration must have been imperfect, as neither were the twenty-four courses of priests complete nor were the Levites in full force.