Exodus 28:43

והיו 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, ועל בניו AND UPON HIS SONS, those prescribed for them. בבאם אל אהל מועד WHEN THEY COME INTO THE APPOINTED TENT — into the Temple, and similarly when they come into the Tabernacle. ומתו 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). חקת 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).

וְהָיוּ֩ עַל־אַהֲרֹ֨ן וְעַל־בָּנָ֜יו בְּבֹאָ֣ם ׀ אֶל־אֹ֣הֶל מוֹעֵ֗ד א֣וֹ בְגִשְׁתָּ֤ם אֶל־הַמִּזְבֵּ֙חַ֙ לְשָׁרֵ֣ת בַּקֹּ֔דֶשׁ וְלֹא־יִשְׂא֥וּ עָוֺ֖ן וָמֵ֑תוּ חֻקַּ֥ת עוֹלָ֛ם ל֖וֹ וּלְזַרְע֥וֹ אַחֲרָֽיו׃ {ס}        

and they shall be upon Aharon, and upon his sons, when they come in the Tent of Meeting, or when they come near to the altar to minister in the holy place; that they bear not iniquity, and die: it shall be a statute for ever to him and his seed after him.