Negative commandment · Lo Ta’aseh #70 of 613
That a priest with a blemish not serve in the Temple
שלא יעבוד בעל מום
The verse · Vayikra 21:17
דַּבֵּר אֶל־אַהֲרֹן לֵאמֹר אִישׁ מִזַּרְעֲךָ לְדֹרֹתָם אֲשֶׁר יִהְיֶה בוֹ מוּם לֹא יִקְרַב לְהַקְרִיב לֶחֶם אֱלֹהָיו
“speak to Aharon, saying, Whoever he be of thy seed in their generations that has any blemish, let him not approach to offer the bread of his God.”
The Rambam’s words
For a priest with a disqualifying physical deformity not to serve [in the Temple], as [Leviticus 21:17] states: "[Anyone...] who has a blemish [may not approach to present]...."
שלא יעבוד בעל מום, שנאמר "אֲשֶׁר יִהְיֶה בוֹ מוּם" (ויקרא כא, יז).
Why · Sefer HaChinuch #275
That a priest with a blemish not serve in the Temple
“It is from the roots of the commandment [that it is] since most activities of people are pleasing to the hearts of their observers based on the importance of the ones doing them.”
Who & when
“And it is practiced at the time of the [Temple] by priests.”
Where in the Mishneh Torah
Hilchot Admission into the Sanctuary הלכות ביאת מקדש