Midrash sur Les Nombres 4:28

6) "of all the mitzvoth of the L–rd": I would understand this as meaning both positive and negative commandments; it is, therefore, written: "which may not be done." (only negative commandments are being referred to.) ("which may not be done" is written four times [Bamidbar 4:2, Bamidbar 4:13, Bamidbar 4:22, Bamidbar 4:28] for four exclusions): I would exclude (from a sin-offering) a lesser positive commandment, but not a greater one (e.g., the eradication of idolatry); it is, therefore, written: "which may not be done." (Only negative commandments are intended.) I would exclude (transgression of) mitzvoth not punishable by kareth, but not pesach and circumcision (transgression of which is) punishable by kareth; it is, therefore, written: "which may not be done." I would exclude pesach, which is not (a) constant (observance), but not circumcision, which is constant; it is, therefore, written: "which may not be done." But then I would exclude the positive commandment of (separation from a niddah (before the time of her flow); it is, therefore, written: "of all the mitzvoth of the L–rd," to include (for a sin-offering one who did not separate and was "surprised" by her flow). Why do you see fit to exclude all (positive) commandments and to include that of niddah? Since Scripture included and excluded, why do I exclude all the (positive) commandments? Because they have no counterpart in a negative commandment. And I include the positive commandment of niddah because it has its counterpart in a negative commandment (viz. [Bamidbar 18:19]: "And to a woman in the niddah state of her uncleanliness you shall not come near.")