David Was Old and Full of Days — 1 Chronicles 23:1–6

Now David was old and full of days; and he made Solomon his son king over Israel. And he gathered together all the princes of Israel, with the priests and the Levites.

And the Levites were numbered from thirty years old and upward: and their number by their polls, man by man, was thirty and eight thousand. Of these, twenty and four thousand were to oversee the work of the house of Jehovah; and six thousand were officers and judges; and four thousand were doorkeepers; and four thousand praised Jehovah with the instruments which I made, said David, to praise therewith.

And David divided them into courses according to the sons of Levi: Gershon, Kohath, and Merari.