Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 71
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 71, with English translation by Maimonides (Rambam).
שלא יעבוד בעל מום עובר, שנאמר "כָּל אִישׁ אֲשֶׁר בּוֹ מוּם לֹא יִקְרַב" (ויקרא כא, יח).
For a priest with a disqualifying physical deformity of a temporary nature not to serve [in the Temple], as [Leviticus 21:18] states: "Anyone who has a blemish may not approach [to offer a sacrifice].