הלכה על שמות 29:9

That is that He commanded the priests to wear special garments for glory and adornment, so they can then serve in the Temple. And that is His saying, "And make holy garments for your brother Aharon, for glory and adornment" (Exodus 28:2); "Then bring his sons forward; clothe them with tunics" (Exodus 29:8). And this is the commandment of the priestly garments - eight garments for the high priests and four for an ordinary priest. And anytime the priest serves with less than this number of special garments, or more than them, his service is disqualified and he becomes liable for death at the hands of the Heavens - meaning for the one was lacking clothes and served. And likewise did they count him in the Gemara (Sanhedrin 83b) as one of those liable for death at the hands of the Heavens. And this explanation does not appear in Scripture. But what does appear in Scripture is, "and you shall gird them with sashes [...] and they shall have priesthood" (Exodus 29:9). And the explanation appears - when their garments are upon them, their priesthood is upon them; when their garments are not upon them, their priesthood is not upon them, and they are outsiders (non-priests). Behold it has been made clear to you that an outsider that serves [in the Temple receives] the death penalty. And they said in the Sifra (Sifra, Tzav, Mechilta d'Milium 1:7), "'And he placed the breastplate upon him': This section was learned for its time and for [all the] generations; for the daily service and for the Yom Kippur service. [However] every day he serves in the golden garments; and on Yom Kippur, in the white (linen) garments. And it already appears in the [Sifra], that wearing these garments is a positive commandment. And this is their saying (Sifra, Acharei Mot, Chapter 8:10), "From where [do we know] that Aharon did not wear the garments for his aggrandizement, but only to fulfill the decree of the King? As it is stated, 'and he did as the Lord commanded Moshe' - that is to say, the wearing of the garments." And even though they are the utmost in beauty - given that they are from gold, onyx, jasper and the other precious and beautiful stones - he should not have intention [in wearing them,] for their beauty, but rather only to fulfill the command that God, may He be exalted, commanded Moshe. And that is that he always wear these garments in the Temple. And the regulations of this commandment have already been explained in Zevachim, Yoma and Sukkah. (See Parashat Tetzaveh; Mishneh Torah, Vessels of the Sanctuary and Those who Serve Therein 10.)

And [the garments] of the high priest are eight, and their names are like this: robe (ketonet), trousers (mikhnasayim) and sash (avnet) - like the names of the three of the regular priest - and mitsnefet (miter) was for the high priest instead of the turban of the regular priest. As this and that were [both] placed on the head, except that the the miter is made like a long type of cloth that women coil around their heads - and the high priest would coil himself with it - but the turban is made like a type of hat (that does not require coiling). Behold, [these] four of the high priest, which were only of linen, were white, six-stranded and embroidered, but their embroidery was not similar to the embroidery of the regular priest's sash. And he also had four others of gold and their names were breastplate (choshen), apron (ephod), coat (me'il) [and] headband (tsits). And he would do the external service with all of the eight, but inside - which is past the partition - he would never serve with anything but the [linen] clothes. And after he served with them for one Yom Kippur, he does not repeat to ever serve with them (Yoma 24a), as it is stated (Leviticus 16:3), "and he leaves them there." And anytime a priest - whether a regular or a high [priest] - serves with less than the clothes that are designated for that service, or more than them, his service is disqualified. And he is also liable for the death penalty by the hand of the Heavens, as our rabbis, may their memory be blessed, learned (Sanhedrin 83b, Zevachim 17b-18a) from "And you shall gird them with a sash [...] and they would have priesthood" (Exodus 29:9) - at the time when their clothes are upon them, their priesthood is upon them; when their clothes are not upon them, their priesthood is not upon them. And they are [hence] considered like a stranger (a non-priest) who transgresses, which is [punishable] by death. [These] and the rest of its details are elucidated in the second chapter of Zevachim and in places in Yoma and Sukkah (see Mishneh Torah, Laws of Vessels of the Sanctuary and Those who Serve Therein 10).