הלכה על בכורות 60:54

From the laws of the commandment is that which they, may their memory be blessed, said (Bekhorot 30b), that we do not accept a [Levite] who accepts all of the commandments of the Levites except for one thing, [but rather only if] he accepts all of them. And their service was that they would be guardsmen for the Temple, as we have written in this Order (Sefer HaChinukh 388). And some of them were gatekeepers, to open and close the gates of the Temple.