Original Verse:
1st Chronicles Chapter 23 Verse 6And David divided them into courses according to the sons of Levi: Gershon, Kohath, and Merari.

Reference Verses:
2nd Chronicles Chapter 8 Verse 14And he appointed, according to the ordinance of David his father, the courses of the priests to their service, and the Levites to their offices, to praise, and to minister before the priests, as the duty of every day required; the doorkeepers also by their courses at every gate: for so had David the man of God commanded.
2nd Chronicles Chapter 35 Verse 4And prepare yourselves after your fathers` houses by your courses, according to the writing of David king of Israel, and according to the writing of Solomon his son.
Ezra Chapter 6 Verse 18And 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.
1st Chronicles Chapter 6 Verse 1The sons of Levi: Gershon, Kohath, and Merari.