Musar zu Dewarim 18:7

Commandment 394 requires the Levites to perform the daily service in the Sanctuary by singing and accompanying the sacrificial service with music (compare 18,23, ועבד הלוי הוא את עבודת אהל מועד, וכו'). The same commandment is repeated in different language in Deut. 18,7: "He shall perform service in the name of the Lord His G–d, just like all his brothers the Levites who stand there in the presence of the Lord." The Talmud in Erchin 11 says that the service referred to is singing.