Midrash su Numeri 4:13

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.")

10) Whence is it derived that nothing is to take precedence to the morning tamid? From "upon it the burnt-offering," (implying that nothing is to take precedence to it). Whence is it to be derived that nothing is offered up later than the afternoon tamid? From "upon it (the afternoon tamid, the shelamim, lit., the "completers," i.e., complete all of the offerings with it). (Vayikra 6:6) ("A continuous fire shall burn upon the altar; it shall not be extinguished.") "continuous" — (The wood pile is to be made for the temidim and the mussafim) even on the Sabbath; "continuous" — even (if the Cohanim are) in a state of tumah. "it shall not be extinguished" — even during their journeyings. What did they do (to keep the fire from going out)? They inverted a psachter (a large vessel) over it. These are the words of R. Yehudah. R. Shimon says: In their journeyings they would remove the fire from the altar (and place it in a vessel until they camped), as it is written (Bamidbar 4:13): "And they shall remove the fire from the altar and spread upon it (the vessel) a purple cloth."