Halakhah к Бамидбар 18:22

That He prohibited an outsider (zar) from serving in the Temple - meaning to say, any man that is not of the offspring of Aharon. And that is His, may He be exalted, saying, "but an outsider shall not approach" (Numbers 18:4). And Scripture explains that one who transgresses this negative commandment is liable for death at the hands of the Heavens. And that is His, may He be exalted, saying, "and an outsider that approaches shall be killed" (Numbers 18:7). And the language of the Sifei (Sifrei Bamidbar 116:2) is, "We have heard of the punishment for service. From where [do we know] the prohibition? [Hence] we learn to say, 'but an outsider should not approach.'" And the prohibition and punishment of this matter have already been repeated; and that is His saying, "And the children of Israel shall no more approach the Tent of Meeting, to bear sin, to die" (Numbers 18:22). And the [types of] service for which an outsider is liable for death have already been explained in the Gemara, Yoma (Yoma 24b). And these are them: Sprinkling [the blood]; burning incense; the water libations; and the wine libations. And the regulations of this commandment have already been explained there and in the second chapter of Zevachim. (See Parashat Korach; Mishneh Torah, Admission into the Sanctuary 9.)

That a foreigner not serve in the Temple: That a foreigner not serve in the Temple - meaning to say anyone who is not from the seed of Aharon - as it is stated (Numbers 18:4), "and a foreigner will not approach you." And the prevention of this is repeated with a different expression in Scripture, and that is as it is stated (Numbers 18:22), "And the Children of Israel shall not again approach the Tent of Meeting, to carry guilt, to die."