הלכה על במדבר 4:19

That He prohibited the Levites from occupying themselves with any of the service that was designated for the priests; and also the priests from occupying themselves with any of the service that was designated for the Levites. And that is because there is a designated service in the Temple for each one of these families. And the prohibition to both of them comes together, not to have one of them serve with the service of the other. Rather each group [is to do] what it was commanded, as He said, "each man to his service" (Numbers 4:19). And the negative commandment that appears about them is His saying to the Levites, "but they shall not [approach] the vessels of the sanctuary and the altar" (Numbers 18:3). But afterwards, He went back and spoke to the priests and said, "that neither they, nor you, shall die" - meaning to say that you, like them, are prohibited. Since just like I prohibited them from doing your service - and that is [working with] the vessels of the sanctuary and the altar - so too are you prohibited from doing their service. And the language of the Sifrei (Sifrei Bamidbar 116:1) is, "'They shall not [approach] the vessels of the sanctuary and the altar' - that is the prohibition. From where [do we know] the punishment? 'And they shall not die.' I only [know about] the Levites, that they are punished and prohibited for [appropriating] the service of the priests. From where [do we learn the same for] priests [appropriating] the service of the Levites? [Hence] we learn to say, 'nor you.' And it once happened that Rabbi Yehoshua [b. Chananiah] sought to assist R. Yochanan [b. Godgada], when he (R. Yehoshua) said to him, 'Get back, for you are forfeiting your life! For I am of the gatekeepers and you are of the singers.'" Behold it has been made clear that a Levite who did service in the Temple besides his [own] service is liable for death at the hands of the Heavens. And likewise should the priests not occupy themselves with the service of the Levites. However if they transgressed, they are not [punished] by death, but rather by lashes. And in the Mikhilta: "'The vessels of the sanctuary' - perhaps [even] if they touched them, they would be liable. [Hence] we learn to say, ['but'] - they are [only] liable for service, they are not liable for touching. I only [know] about Levites on account of [the service of] priests. From where [do we know] priests on account of [the service of] Levites? [Hence] we learn to say, 'nor you.'" And there they said, ["Levites on account of priests is with death. But priests on account of Levites is only with a negative commandment."] (See Kessef Mishneh on Mishneh Torah, Vessels of the Sanctuary and Those who Serve Therein 3:9; Parashat Bamidbar, Korach; Mishneh Torah, Vessels of the Sanctuary and Those who Serve Therein 3.)

That the priests not be involved in the service of the the Levites and the Levites in the service of the priests: That the priests not be involved in the service of the Levites and the priests in the business of the Levites, but rather each one does the work designated for him, from that which is written (Numbers 4:19), "each man to his service and his load." And the language of the prevention that comes about this is that which is stated about the Levites (Numbers 18:3), "but to the vessels of the Holy and to the Altar they shall not approach"; and afterwards the verse returns to speak about the priests, "and they shall not die, also they and also you" - meaning to say with this that you too, you are included in this negative commandment. Since just like the Levites are prevented from their work [when it is not assigned to them], so too are you prevented from your work [when it is not assigned to you]. And the language of the Sifrei on Numbers 18:3: "'To the vessels of the Holy and to the Altar' [is the] warning; 'and they shall not die' is the punishment. I only [know] that the Levites are punished and warned about the service of the priests; from where do I know [the same] about the priests [doing] the service of the Levites? [Hence] we learn to say, 'also you.'" And we have found (Sifrei on Numbers 18:3) that Rabbi Yehoshua ben Chananiah wanted to help Rabbi Yochanan ben Gudgodah in the closing of the doors, [and] he said to him, "Go back, as you are already liable with your soul (life), as I am from the gatesmen and you are from the singers." Behold, it is elucidated that any Levite that does work in the Temple that is not his designated work is liable for death by the hand of the Heavens. And so [too,] the priests are warned not to approach the work of the Levites. However, if they did transgress this, they are not killed, but rather [get] lashes. And they said in the Mekhilta (Sifra Zuta), "'To the vessels of the Holy and to the Altar they shall not approach' It is possible that they would be liable if they touched [them]. [Hence] we learn to say, 'but' - for the work they are liable, and not for touching. I only [know] about the Levites for the [work of the] priests, from where do I know about the priests for the [work of the] Levites? [Hence] we learn to say, 'also you.'" And there it is said that the Leivtes on that of the priests is [punished] with death, but the priests on that of the Levites is only with [the consequences of violating] a negative commandment.