Negative commandment · Lo Ta’aseh #71 of 613
That a priest with a transient blemish not serve
שלא יעבוד בעל מום עובר
The verse · Vayikra 21:18
כִּי כׇל־אִישׁ אֲשֶׁר־בּוֹ מוּם לֹא יִקְרָב אִישׁ עִוֵּר אוֹ פִסֵּחַ אוֹ חָרֻם אוֹ שָׂרוּעַ
“For whatever man he be that has a blemish, he shall not approach: a blind man, or a lame, or he that has a flat nose, or anything superfluous,”
The Rambam’s words
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].
שלא יעבוד בעל מום עובר, שנאמר "כָּל אִישׁ אֲשֶׁר בּוֹ מוּם לֹא יִקְרַב" (ויקרא כא, יח).
Who & when
“And [it] is practiced at the time of the [Temple] by the priests.”
Where in the Mishneh Torah
Hilchot Admission into the Sanctuary הלכות ביאת מקדש