Midrash su Numeri 8:3

3b) binyan av mishnei kethuvim (a general rule derived from two verses). viz.: The topic of the lamps (on the menorah) is not like that of sending the unclean outside (of the encampment), and the latter is not like the former. What is common to them is that they are introduced by "Tzav" ("Command") — [the lamps, (Vayikra 24:2); sending, etc., (Bamidbar 5:2)] — and apply both immediately and for future generations. [The lamps: immediately — (Bamidbar 8:3): "And Aaron did so. He kindled its lamps towards the face of the menorah, etc."; for future generations — (Vayikra 24:3): "… an eternal statute throughout your generations." Sending the unclean outside: immediately — (Bamidbar 5:4): "And the children of Israel did so, and they sent them outside the camp"; for future generations — (Bamidbar 19:21): "And it shall be for them an everlasting statute."] So, all commandments introduced by "Tzav" apply both immediately and for future generations.

7) kllal shehu tzarich lifrat ufrat shehu tzarich lichllal (general requiring specific and specific requiring general): (Shemoth 13:2): "Sanctify unto Me every bechor (first-born), the opener of each womb." I might think that even females are included; it is, therefore, written (Devarim 15:19): "the male." If "the male," (I would think that) even a yotze dofen (Caesarian birth) were a bechor. It is, therefore, written "the opener of the womb." This is an instance of "general" (bechor) requiring "specific." I might think that even if it were born after a yotze dofen it were a bechor (being the first opener of the womb); it is, therefore, written "bechor," (which connotes first in all respects; not only first opener of the womb, but also first in birth). This is an instance of "specific" ("opener of the womb") requiring "general" (bechor).

(Bamidbar 8:3) "And Aaron did so": This is in praise of Aaron. As Moses told him, thus did he do, without any change. He made "mul" and "panim" (see above). "He'elah" (lit., "he raised") its lamps" — whence they said: There was an ascent before the menorah of three steps on which the Cohein stood and tended to the lamps, (after which) he placed the oil jug on the second step and left. "as the L-rd had commanded Moses" (i.e., half a log for each lamp, etc.) This tells me only of Aaron (the high-priest). Whence do I derive the same for his sons (i.e., ordinary Cohanim)? From (Vayikra 24:3) "Aaron and his sons shall arrange it." This tells me only of the menorah, that the sons were equated with the father. Whence do I derive the same for the (offering of the) incense? (viz. Shemot 34:7) "It follows, viz.: "Service in the tent of meeting" is written in respect to the menorah, and it is also written in respect to the incense. If I have learned of the first that sons are equated with the father, so, do I learn with the second. — (No,) this is refuted by the service of Yom Kippur, in which instance, even though "service in the tent of meeting" is written in respect to it, the sons are not equated with the father. And this refutes (the argument for) incense, which, even though "service in the tent of meeting" is written thereof, we would not equate the sons with the father. — Would you say that? There is a (strategic) difference! "service in the tent of meeting in golden vestments" is written both in respect to the menorah and in respect to the incense, and this is not to be refuted by the service of Yom Kippur, which, even though "service in the tent of meeting" is written thereof, is not in golden (but in linen vestments). — This (argument) is refuted by the instance of the bullock of "forgetfulness" of the anointed (high-priest [viz. Vayikra 4:3]) whereof "service in the tent of meeting in golden vestments" is written, and in respect to which sons were not equated with the father. And this will refute (the argument for) incense, which even though "service in the tent of meeting in golden vestments" is written thereof, we would not equate the sons with the father. Would you say that? There is a difference! I would derive it from three terms together. In respect to the menorah it is written "service in the tent of meeting," and "golden vestments," and also "continuously" (tamid), and thus is it written of incense. And this is not to be refuted by the service of Yom Kippur, where, even though "service in the tent of meeting" is written thereof, it is not in golden vestments. Nor (is it to be refuted) by the bullock of forgetfulness of the anointed (high-priest), where, even though "service in the tent of meeting in golden vestments" is written thereof, "continuously" is not written thereof. I will learn a thing from a (similar) thing, and I will derive a thing from a (similar) thing. I will learn a thing from another thing which is similar to it in three ways, but not from a thing that is not similar to it in three things, but only in one or two. Therefore, if I have learned in respect to the menorah that sons are equated with the father, so, I will learn in respect to the incense that the sons are equated with the father.