2 Chronicles 35:4

And prepare yourselves
To do their work in this service of the sanctuary, the passover; that they be ready to do it, and diligent in it, and perform it according to the law of God:

by the houses of your fathers, after your courses;
such of them whose turn in course it was to officiate:

according to the writing of David king of Israel, and according to the
writing of Solomon his son;
who had given in writing directions in what manner their courses should be observed, see ( 1 Chronicles 23:1-24:31 ) .