Keritot 25A

אמר רבי ינאי: גבול שמעתי בה ושכחתי, ונסבין חברייא למימר: ירידתה לנחל איתן אוסרתה.

R' Yannai says: I heard an answer with regard to the boundary, i.e., the stage, beyond which it is prohibited, but I have forgotten what it is; and the members of the group of rabbis were inclined to say that its descent to the rough, dried-up stream, where its neck is broken, is the action that renders it prohibited.

אמר רב המנונא: מנא אמינא לה? דתנן: השוחט פרת חטאת, ושור הנסקל, ועגלה ערופה ⁠– רבי שמעון פוטר, וחכמים מחייבין.

Rav Hamnuna said: From where do I say that the prohibition takes effect when the animal is alive? I say it from that which we learned in a Mishnah (Ḥullin 81b): With regard to one who slaughters the red heifer of purification, or an ox that was sentenced to be stoned, or a heifer whose neck is broken, all of which are animals from which deriving benefit is prohibited, R' Shimon exempts one who slaughters them from receiving lashes for violating the prohibition of the slaughter of a mother and its offspring, if the mother was slaughtered on that same day; and the Rabbis deem him liable.

בשלמא לדידי, דאמינא מחיים, בהא פליגי רבי שמעון ורבנן, דרבי שמעון סבר: שחיטה שאינה ראויה ⁠– לא שמה שחיטה.

Rav Hamnuna explains his proof: Granted, according to my opinion, that I say the prohibition takes effect from the time when the animal is alive, R' Shimon and the Rabbis disagree with regard to this matter, as R' Shimon holds: Slaughter that does not render the animal fit for consumption is not considered slaughter. The slaughter of the heifer does not render it fit for consumption, as it was already prohibited to derive benefit from the animal while it is alive. Therefore, he does not violate the prohibition against slaughtering a mother and its offspring.