The Priestly Garments
Requires Beit HaMikdash
Shemot 28:2 You shall make vestments of sanctity for Aharon your brother, for glory and splendor.
The Kohanim are commanded to array themselves in garments of special splendor and beauty before they minister in the Temple.
These are the Priestly Garments: eight garments for the Kohen Gadol, and four for a common Kohen. Whenever a Kohen performs the service with fewer or with more than the number of garments appointed for that particular service, the service is invalid, and he is also liable to death by the hand of Heaven.1
1. In his Mishneh Torah, Maimonides clearly expresses the opinion that the same penalty is incurred by a Kohen who performs the service with more garments than the number appointed (Mishneh Torah, Avodah, Hilchot Kle Ha-Mikdash 10, 4)