Rashi on Exodus 28:43/רש"י על שמות כ״ח:מ״ג

1והיו על אהרן AND THEY SHALL BE UPON AARON — “they” means all these garments (not only the breeches which are the last-mentioned garments); upon Aaron shall be those which are proper to him,

2ועל בניו AND UPON HIS SONS, those prescribed for them.

3בבאם אל אהל מועד WHEN THEY COME INTO THE APPOINTED TENT — into the Temple, and similarly when they come into the Tabernacle.

4ומתו AND THEY DIE — thus you may learn that he who officiates lacking any of these garments is liable to death (Midrash Tanchuma, Achrei Mot 6).

5חקת עולם לו AN ORDINANCE TO HIM FOR EVER — Wherever it is said “an ordinance forever” it is an enactment for the immediate present time and for future generations, and the phrase is used to make invalid thereby (through the fact that this phrase is used) any rite where the details prescribed are not fully carried out (cf. Menachot 19a).