2 Chronicles 31:17

and them that were reckoned by genealogy of the priests by their fathers houses, and the Levites from twenty years old and upward, in their offices by their courses;

1 Chronicles 23:24

These were the sons of Levi after their fathers houses, even the heads of the fathers houses of those of them that were counted, in the number of names by their polls, who did the work for the service of the house of Jehovah, from twenty years old and upward.

1 Chronicles 23:27

For by the last words of David the sons of Levi were numbered, from twenty years old and upward.

Numbers 3:15

Number the children of Levi by their fathers houses, by their families: every male from a month old and upward shalt thou number them.

Numbers 3:20

And the sons of Merari by their families: Mahli and Mushi. These are the families of the Levites according to their fathers houses.

Numbers 4:3

from thirty years old and upward even until fifty years old, all that enter upon the service, to do the work in the tent of meeting.

Numbers 4:38

And those that were numbered of the sons of Gershon, their families, and by their fathers houses,

Numbers 4:42

And those that were numbered of the families of the sons of Merari, by their families, by their fathers houses,

Numbers 4:46

All those that were numbered of the Levites, whom Moses and Aaron and the princes of Israel numbered, by their families, and by their fathers houses,

Numbers 8:24

This is that which belongeth unto the Levites: from twenty and five years old and upward they shall go in to wait upon the service in the work of the tent of meeting:

Numbers 17:2-3

Speak unto the children of Israel, and take of them rods, one for each fathers house, of all their princes according to their fathers houses, twelve rods: write thou every man's name upon his rod.

1 Chronicles 24:20-26

And of the rest of the sons of Levi: of the sons of Amram, Shubael; of the sons of Shubael, Jehdeiah.

2 Chronicles 31:2

And Hezekiah appointed the courses of the priests and the Levites after their courses, every man according to his service, both the priests and the Levites, for burnt-offerings and for peace-offerings, to minister, and to give thanks, and to praise in the gates of the camp of Jehovah.

Ezra 2:59

And these were they that went up from Tel-melah, Tel-harsha, Cherub, Addan, and Immer; but they could not show their fathers houses, and their seed, whether they were of Israel:

Treasury of Scripture Knowledge

Summary

Both to the genealogy of the priests by the house of their fathers, and the Levites from twenty years old and upward, in their charges by their courses;

Bible References

Genealogy

Numbers 3:15
Number the children of Levi by their fathers houses, by their families: every male from a month old and upward shalt thou number them.
Numbers 4:38
And those that were numbered of the sons of Gershon, their families, and by their fathers houses,
Numbers 17:2
Speak unto the children of Israel, and take of them rods, one for each fathers house, of all their princes according to their fathers houses, twelve rods: write thou every man's name upon his rod.
Ezra 2:59
And these were they that went up from Tel-melah, Tel-harsha, Cherub, Addan, and Immer; but they could not show their fathers houses, and their seed, whether they were of Israel:

Twenty

Numbers 4:3
from thirty years old and upward even until fifty years old, all that enter upon the service, to do the work in the tent of meeting.
Numbers 8:24
This is that which belongeth unto the Levites: from twenty and five years old and upward they shall go in to wait upon the service in the work of the tent of meeting:
1 Chronicles 23:24
These were the sons of Levi after their fathers houses, even the heads of the fathers houses of those of them that were counted, in the number of names by their polls, who did the work for the service of the house of Jehovah, from twenty years old and upward.

By their courses

2 Chronicles 31:2
And Hezekiah appointed the courses of the priests and the Levites after their courses, every man according to his service, both the priests and the Levites, for burnt-offerings and for peace-offerings, to minister, and to give thanks, and to praise in the gates of the camp of Jehovah.
1 Chronicles 24:20
And of the rest of the sons of Levi: of the sons of Amram, Shubael; of the sons of Shubael, Jehdeiah.

American Standard Version Public Domain