Sanhedrin 73A

מתני׳ ואלו הן שמצילין אותן בנפשן: הרודף אחר חבירו להרגו, ואחר הזכר, ואחר הנערה המאורסה. אבל הרודף אחר בהמה, והמחלל את השבת, ועובד עבודה זרה ⁠– אין מצילין אותן בנפשן.

Mishnah: And these are the ones who are saved from transgressing even at the cost of their lives; that is to say, these people may be killed so that they do not perform a transgression: One who pursues another to kill him, or pursues a male to sodomize him, or pursues a betrothed young woman to rape her. But with regard to one who pursues an animal to sodomize it, or one who seeks to desecrate Shabbat, or one who is going to engage in idol worship, they are not saved at the cost of their lives. Rather, they are forewarned not to transgress, and if they proceed to transgress after having been forewarned, they are brought to trial, and if they are found guilty, they are executed.

גמ׳ תנו רבנן: מניין לרודף אחר חבירו להרגו שניתן להצילו בנפשו? תלמוד לומר: ״לא תעמד על דם רעך״. והא להכי הוא דאתא? האי מיבעי ליה לכדתניא: מניין לרואה את חבירו שהוא טובע בנהר, או חיה גוררתו, או לסטין באין עליו, שהוא חייב להצילו? תלמוד לומר: ״לא תעמד על דם רעך״. אין, הכי נמי.

Talmud: A baraita states: From where is it derived that with regard to one who pursues another in order to kill him, the pursued party may be saved at the cost of the pursuer’s life? The verse states: “You should not stand idly by the blood of another” (Leviticus 19:16); rather, you must save him from death. The Talmud asks: But does this verse really come to teach us this? This verse is required for that which is taught in a baraita: From where is it derived that one who sees another drowning in a river, or being dragged away by a wild animal, or being attacked by bandits [listin], is obligated to save him? The Torah states: “You should not stand idly by the blood of another.” The Talmud answers: Yes, it is indeed so that this verse relates to the obligation to save one whose life is in danger.