Daily Mitzvah (Rambam)

The 167th prohibition is that a kohen gadol is forbidden from being under the same roof as a dead body, even if it is someone who there is a command to mourn, i.e. a close relative.1

The source of this prohibition is G‑d's statement,2 "He shall not come into [a tent containing] any dead body."

If he makes himself tameh — even for his father or mother — he is punished by lashes.

Footnotes

1.

I.e. father, mother, son, daughter, brother or sister.

2.

Lev. 21:11.