הלכה על במדבר 18:5

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.