The 23rd mitzvah is the commandment to the Levites to carry out certain types of service in the Holy Temple, such as closing the gates, and singing at the time when the sacrifices were offered.
The source of this mitzvah is G‑d's statement,1 "This is the service of the Levites."
The Sifri2 says, "We might think that the Levite can serve if he wants to, but that if he doesn't want to, he is not obligated. The Torah therefore says, 'This is the service of the Levites' — even against their will." This means that it is obligatory upon him and that the mitzvah is forced upon him.
What the service of the Levites consists of is explained in many places in tractates Tamid3 and Middos.4 It is also explained in the second chapter of tractate Erachin5 that only the Levites can perform the service of singing. This commandment is repeated in another verse,6 "He can serve in the name of G‑d, his G‑d, just the same as any of his fellow Levites." Our Sages explained in the second chapter of Erachin, "What is meant by service 'in the name of G‑d'? It refers to the singing."
