Sforno on Leviticus 21:18/ספורנו על ויקרא כ״א:י״ח

1כי כל איש אשר בו מום לא יקרב, to stand and perform the Temple service. The principle is familiar to us from when Esther was aghast when her uncle Mordechai appeared wearing sackcloth in the courtyard of the Royal Palace (Esther 4,2).