Positive commandment · Mitzvat Aseh #32 of 613
Sanctifying the seed of Aharon
לחלוק כבוד לזרעו של אהרן, ולהקדימו לכל דבר שבקדושה
The verse · Vayikra 21:8
וְקִדַּשְׁתּוֹ כִּי־אֶת־לֶחֶם אֱלֹהֶיךָ הוּא מַקְרִיב קָדֹשׁ יִהְיֶה־לָּךְ כִּי קָדוֹשׁ אֲנִי ה׳ מְקַדִּשְׁכֶם
“Thou shalt sanctify him therefore; for he offers the bread of thy God: he shall be holy to thee: for I the Lord, who sanctify you, am holy.”
The Rambam’s words
To show honor to Aaron's descendants and to give them priority regarding all holy matters, as [Leviticus 21:8] states: "And you shall sanctify him."
לחלוק כבוד לזרעו של אהרן, ולהקדימו לכל דבר שבקדושה, שנאמר: "וְקִדַּשְׁתּוֹ" (ויקרא כא, ח).
Why · Sefer HaChinuch #269
Sanctifying the seed of Aharon
“It is from the roots of the commandment [that it is] since it is known that honoring his servants is from the honor of the master. And every time we honor the priests, we remember and fix His honor, blessed be He, and His greatness in our thoughts.”
Who & when
“And [it] is practiced in every place and at all times by males and females, as it is a commandment upon all of them to honor the seed of Aharon.”
Where in the Mishneh Torah
Hilchot Vessels of the Sanctuary and Those Who Serve Therein הלכות כלי המקדש והעובדין בו