Positive commandment · Mitzvat Aseh #33 of 613
To clothe the priests with the priestly garments for service [in the Temple]
להיות הכהנים לובשים לעבודה בגדי כהונה
The verse · Shemot 28:2
וְעָשִׂיתָ בִגְדֵי־קֹדֶשׁ לְאַהֲרֹן אָחִיךָ לְכָבוֹד וּלְתִפְאָרֶת
“And thou shalt make holy garments for Aharon thy brother for honour and for beauty.”
The Rambam’s words
To clothe the priests with the priestly garments for service [in the Temple], as [Exodus 28:2] states: "And you shall make holy garments...."
להיות הכהנים לובשים לעבודה בגדי כהונה, שנאמר: "וְעָשִׂיתָ בִגְדֵי קֹדֶשׁ..." (שמות כח, ב).
Why · Sefer HaChinuch #99
Wearing the priestly clothes
“From the roots of the commandment is the principle established for us, that a person is impacted according to his actions and pursuant to his thoughts and intentions.”
Who & when
“And this commandment is practiced at the time of the Temple by males of the priesthood.”
Where in the Mishneh Torah
Hilchot Vessels of the Sanctuary and Those Who Serve Therein הלכות כלי המקדש והעובדין בו