Negative commandment · Lo Ta’aseh #167 of 613
That a high priest not enter the tent of a dead body
שלא ייטמא כוהן גדול ואפילו לקרוביו
The verse · Vayikra 21:11
וְעַל כׇּל־נַפְשֹׁת מֵת לֹא יָבֹא לְאָבִיו וּלְאִמּוֹ לֹא יִטַּמָּא
“neither shall he go in to any dead body, nor defile himself for his father, or for his mother;”
The Rambam’s words
For a High Priest not to become impure [through contact with any corpse], even [those of] his relatives, as [Leviticus 21:11] states: "He shall not become impure, [even] for his father and mother."
שלא ייטמא כוהן גדול ואפילו לקרוביו, שנאמר "לאביו ולאימו, לא ייטמא" (ויקרא כא,יא).
Why · Sefer HaChinuch #270
That a high priest not enter the tent of a dead body
“It is from the roots of the commandment [that] since the priests were chosen for the service of G-d, blessed be He — as the verse stated, “they shall be holy to their G-d” (Leviticus 21:6) — therefore, He distanced them from the dead.”
Who & when
“And this prohibition is practiced at the time of the [Temple] when there was a High Priest there.”
Where in the Mishneh Torah
Hilchot Mourning הלכות אבל