Parashas Acharei Mos · Passuk 18:05
Chabad.org — English translation + Rashi
Torah Temima points to Sanhedrin 74a.
English: They voted in the loft of Beit Nitzah in Lod: for all mitzvot except idolatry, forbidden relations and murder — if told “transgress or be killed,” one should transgress and not be killed; the reason is “live by them.”
Yehareg ve’al ya’avor exceptions
“Live by them” establishes the general rule of pikuach nefesh; the three cardinal sins remain absolute even under threat of death.