1 Chronicles 25:1
Moreover David and the captains of the host separated to the service of the sons of Asaph, and of Heman, and of Jeduthun, who should prophesy with harps, with psalteries, and with cymbals: and the number of the workmen according to their service was:
David joined with the captains of the host in setting apart the sons of Asaph, Heman, and Jeduthun to the service of music. Those appointed were to prophesy with harps, with psalteries or lyres, and with cymbals. The workmen were numbered and organized according to their service, with duties assigned and counted according to each group's work. They were divided into twenty-four courses, each under twelve men trained and skilled in playing instruments.