Sefer HaMitzvot · Negative Commandments, Chapter 188
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הזהירנו מאכול בשר שור הנסקל ואפילו נשחטהו קודם שנסקל שהוא מעת שנגמר דינו נאסרה אכילתו אף על פי שנשחט שחיטה כשרה, והוא אמרו לא יאכל את בשרו. ולשון מכילתא שור שיצא ליסקל וקדמו הבעלים ושחטוהו בשרו אסור באכילה לכך נאמר לא יאכל. ומי שאכל ממנו כזית לוקה. (שם, שם):
He prohibited us from eating meat from an ox that is to be stoned. And even if it was slaughtered before it was stoned - from the time its judgement was completed - it is forbidden [to] eat it, even though it was slaughtered with a proper slaughter. And that is His saying, "its meat shall not be eaten" (Exodus 21:28). And the language of the Mekhilta (Mekhilta d'Rabbi Yishmael 21:28:3) is, "[If] the ox were taken out to be stoned, and its owners intervened and slaughtered it, its meat is forbidden to eat." Hence it is stated, "it shall not be eaten." And one who eats a kazayit of it is lashed. (See Parashat Mishpatim; Mishneh Torah, Forbidden Foods 9.)
הזהירנו מאכול בשר שור הנסקל ואפילו נשחטהו קודם שנסקל שהוא מעת שנגמר דינו נאסרה אכילתו אף על פי שנשחט שחיטה כשרה, והוא אמרו לא יאכל את בשרו. ולשון מכילתא שור שיצא ליסקל וקדמו הבעלים ושחטוהו בשרו אסור באכילה לכך נאמר לא יאכל. ומי שאכל ממנו כזית לוקה. (שם, שם):
He prohibited us from eating meat from an ox that is to be stoned. And even if it was slaughtered before it was stoned - from the time its judgement was completed - it is forbidden [to] eat it, even though it was slaughtered with a proper slaughter. And that is His saying, "its meat shall not be eaten" (Exodus 21:28). And the language of the Mekhilta (Mekhilta d'Rabbi Yishmael 21:28:3) is, "[If] the ox were taken out to be stoned, and its owners intervened and slaughtered it, its meat is forbidden to eat." Hence it is stated, "it shall not be eaten." And one who eats a kazayit of it is lashed. (See Parashat Mishpatim; Mishneh Torah, Forbidden Foods 9.)