Les Nombres 18:23 Halakha : Mishneh Torah, Foundations, Mishneh Torah, Positive Mitzvot & Sefer HaChinukh

It is a clear and manifest principle concerning the Torah that as a Law it is permanently established forever and evermore; and that it is not subject to mutibility, nor to diminution, nor to amendment; for it is said: "All this word which I command you, that shall ye observe to do; thou shalt not add thereto, nor diminish from it" (Ibid. 13.1); and it is further said: "But the things that are revealed belong unto us and to our children forever, that we may do all the words of this law" (Ibid. 29.28). Herefrom we learn that according to all the words of the Torah we are commanded to do forever. It is, moreover, said: "a statute forever throughout your generations" (Num. 15.15); and again it is said:¹See Baba Mezi’a 59. C. "It is not in heaven" (Deut. 30.12); herefrom we learn that no prophet is thereafter permitted to make any innovation. Therefore, should a man rise up, whether he be from among the Gentiles or whether from among Israel, and deliver a token and perform a miracle saying, that God hath sent him to add a commandment, or to diminish a commandment, or to interpret a certain commandment of among the commandments with such interpretation as we have not heard by tradition from Moses; or he said, that these commandments with which Israel was charged are not forever and throughout all generations, but that they were commandments in keeping with those times only, behold him, he is a false prophet, seeing that he came to deny the prophecy of Moses; and his punishment is death by strangulation, because he wilfully spoke in the name of God concerning that which He did not charge him; for He, blessed is He! commanded Moses, saying, that this enactment was "unto us and unto our children forever" (Deut. 29.28); and, "God is not a man that He should lie" (Num. 23.19).

That the Levite shall serve in the Sanctuary, as it is said, “But the Levite shall do the service of the tabernacle” (Num. 18:23).

The commandment of the service of the Levites in the Temple: That the Levites serve in the Temple to be gatekeepers and to sing everyday over the sacrifice, as it is stated (Numbers 18:23), “And the Levite shall serve, he, etc.” And the language of the Sifrei is “I might understand [it to mean:] if he wants, he serves, and if he does not want, he does not serve. [Hence], we learn to say, ‘And the Levite shall serve, he’ — [even] against his will,” meaning to say that this thing is an obligation, and that this matter is incumbent upon him perforce. And it is also elucidated in the second chapter of Arakhin 11a, that no one should ever say the song of the mouth (in the Temple) besides the Levite. And this commandment is repeated in the Torah with a different expression, as it is stated (Deuteronomy 18:7), “And he shall serve in the name of the Lord, his God.” And they said there in Arakhin, “What is the service that is [done] in the name of the Lord? It should be said, it is song.”