הלכה על במדבר 18:4

The commandment to guard the Temple: That the priests and the Levites were commanded to guard the Temple and walk around it constantly the whole night, on each and every night (see R. Chaim Heller - Sefer HaMitzvot LaRambam, Mitzvot Ase 22) - and this guarding is to honor it, to exalt it and to glorify it, and not from there being any fear from the enemy, God forbid - as it is stated (Numbers 18:4), "and they shall guard the guarding of the Tent of Meeting." And the language of Sifrei Bamidbar 116 is "'And you and your sons with you will be in front of the Tent of Testimony' (Numbers 18:2) - the priests inside and the Levites only outside"; meaning to say to guard it and to walk around it. And in the Mekhilta (Sifrei Zuta on Numbers 18:4), they said, "'And they shall guard the guarding of the Tent of Meeting' - I only know of a positive commandment, etc." Behold, it is elucidated that its guarding is a positive commandment. And there it says another greatness of the Temple is that it has guards - "A palace that has guards is not similar to a palace that does not have guards." And it is known that the palace is a name for the sanctuary.

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."

To not nullify the guarding of the Temple: To not nullify the guarding of the Temple, [but rather] to always walk around it every night, as it is stated (Numbers 18:5), "And you shall guard the guarding of the Holy." And it is well-known that the expression of guarding takes the place of a negative commandment. And [it is] as they, may their memory be blessed, said (Eruvin 96a), "Every place in which it is stated, 'guard yourself,' 'lest,' and 'not,' is nothing but a negative commandment." And they said in Mekhilta, "'And they shall guard the guarding of the Tent of Meeting' (Numbers 18:4), - I only [know of] a positive commandment. From where [do I know] a negative commandment? [Hence], we learn to say, 'And you shall guard the guarding of the Holy.'" To here [are the words of Mekhilta]. And maybe they expounded a positive commandment and a negative commandment when they found two verses teaching one thing. And from the one that comes by way of a command in second person, they learned a negative commandment; and they expounded the command in third person - which is lighter than it - about a positive commandment. And anyone who weighs the things in just scales will concede about this - that the third person is lighter than a command in second person.