Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 69
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 69, with English translation by Maimonides (Rambam).
שלא יכנס בעל מום מן המזבח ולפנים, שנאמר "אַךְ אֶל הַפָּרֹכֶת לֹא יָבֹא" (ויקרא כא, כג).
For a priest with a disqualifying physical deformity not to proceed beyond [the beginning of] the altar, as [Leviticus 21:23] states: "However, to the parochet he shall not approach."