Halakhah zu Bamidbar 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 can understand 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."