Lex Dei moralis, ceremonialis, et politica, ex libris Mosis excerpta, et in certas classes distributa

14 Furthermore, Iehouah spoke to Moses in the desert of Sinai, saying,
15 Number the sons of Levi by the houses of their fathers, by their families: every male from one month and upward, you shall number.
16 And so Moses numbered them according to the word of Iehouah, just as he had been commanded.
39 All the numbered ones of the Levites, whom Moses and Aaron numbered according to the word of Iehouah by their families, all the males from one month and upward, were twenty-two thousand.
40 And Iehouah said to Moses, Number every firstborn male of the sons of Israel from one month and upward, and take the number of their names.
41 And you shall take the Levites for me (I am Iehouah) in place of all the firstborn of the sons of Israel, and the beasts of the Levites in place of all the firstborn of the beasts of the sons of Israel.
42 And so Moses numbered, just as Iehouah had commanded him, every firstborn among the sons of Israel.
43 And all the firstborn males, according to the number of names, from one month old and upward, by their numbered ones, were twenty-two thousand two hundred and seventy-three.
44 And Iehouah spoke to Moses, saying,
45 Take the Levites in place of all the firstborn among the sons of Israel, and the beasts of the Levites for their beasts, and the Levites shall be mine. I am Iehouah.
46 Furthermore, the redemptions of the two hundred and seventy-three which are redundant beyond the Levites: from the firstborn of the sons of Israel.
47 You shall take five shekels for each head, according to the shekel of the sanctuary you shall take: twenty obols a small unit of weight/currency is a shekel.
48 And you shall give that money to Aaron and his sons, the redemptions of those who are in excess among them.
49 Therefore Moses took the redemption money from those who were in excess beyond those redeemed by the Levites:
50 From the firstborn of the sons of Israel he took money, one thousand three hundred and sixty-five shekels according to the shekel of holiness.
51 And Moses gave the redemption money to Aaron and his sons, according to the word of Iehouah, just as Iehouah had commanded him.
Ch. 8, v. 14 And you shall separate the Levites from the midst of the sons of Israel, the Levites shall be mine.
15 After this, however, the Levites shall come to minister in the tabernacle of the congregation, and you shall purify them, and you shall offer them as an offering:
16 Because they are given, they are given, I say, to me from the midst of the sons of Israel, for the one opening every womb, for the firstborn of all the sons of Israel I have taken them for myself.
17 For every firstborn among the sons of Israel is mine, as well of men as of beasts: from the day on which I struck every firstborn in the land of Egypt, I sanctified them for myself.
18 And I have taken the Levites for every firstborn among the sons of Israel.
19 And I have given the Levites as a gift given to Aaron and his sons from the midst of the sons of Israel, so that they may perform the office of the sons of Israel in the tabernacle of the congregation, so that they may perform their duty: and there shall not be a plague in Israel, when they shall approach the sanctuary.
23 Furthermore, Iehouah spoke to Moses, saying,
24 This also is what pertains to the Levites, from the twenty-fifth year and upward they shall enter to wage the warfare in the service of the tabernacle of the congregation.
25 From the fiftieth year he shall return from the warfare of service, and shall not minister any further:
26 But he shall minister with his brethren in the tabernacle of the congregation, so that they may perform their duty: but he shall not administer the ministry. Thus you shall do concerning the Levites in their duties.

Num. ch. 3, verse 6.

AND you shall appoint Aaron and his sons, and they shall guard their priesthood: and an outsider indeed who shall approach, shall die.
Ch. 18, v. 1 And Iehouah said to Aaron, You, and your sons, and the house of your father with you shall bear the iniquity of the Sanctuary: you also and your sons with you shall bear the iniquity of your priesthood.
2 And also your brethren, the tribe of Levi, the tribe of your father, cause to approach to you, and let them adhere to you, and let them minister to you: but you and your sons with you shall stand before the tabernacle of the testimony.