Bava Kamma 46A

אבל מועד לגמרי ⁠– לא משכחת ביה צד תמות כלל.

But if an ox is entirely rendered forewarned, you do not find an element of innocuousness with regard to it at all. If it is forewarned with regard to goring with either horn, its status of innocuousness is completely revoked.

רבי אליעזר אומר: אין לו שמירה אלא סכין (כו׳). אמר רבה: מאי טעמא דרבי אליעזר? דאמר קרא: ״ולא ישמרנו״⁠ – שוב אין לו שמירה לזה.

§ The Mishnah teaches that R' Eliezer says: An ox has no sufficient safeguarding at all other than slaughtering it with a knife. Rabba said: What is the reason for the opinion of R' Eliezer? It is as the verse states with regard to a forewarned ox: “And the owner has not secured it” (Exodus 21:36), meaning that once it is rendered forewarned the owner no longer has any sufficient manner of safeguarding this animal, and the owner is responsible for all damage it causes.

אמר ליה אביי: אלא מעתה, דכתיב ״ולא יכסנו״ נמי ⁠– שוב אין לו כיסוי לזה?

Abaye said to Rabba: If that is so, does that which is written with regard to a pit: “And if a man should open a pit, or if a man should dig a pit and not cover it” (Exodus 21:33), also mean that once a pit has been dug the owner no longer has any adequate way of covering this pit, which would exempt its owner from paying damages?

וכי תימא הכי נמי, והתנן: כסהו כראוי, ונפל לתוכו שור או חמור ומת ⁠– פטור.

And if you would say that indeed that is the halakha, but didn’t we learn in a Mishnah (52a) that if he covered the pit appropriately, and an ox or a donkey fell into it and died, he is exempt? Evidently, a pit can be covered adequately.

אלא אמר אביי: היינו טעמיה דרבי אליעזר ⁠– כדתניא, רבי נתן אומר: מניין שלא יגדל אדם כלב רע בתוך ביתו, ואל יעמיד סולם רעוע בתוך ביתו? שנאמר: ״ולא תשים דמים בביתך״.

Rather, Abaye rejected Rabba’s explanation of R' Eliezer’s opinion, and said that this is the reason for the opinion of R' Eliezer: As it is taught in a baraita that R' Natan says: From where is it derived that one may not raise a vicious dog in his house, and that one may not set up an unstable ladder in his house? As it is stated: “You should not bring blood into your house” (Deuteronomy 22:8), which means that one may not allow a hazardous situation to remain in his house. Similarly, a person should not keep a forewarned ox in his possession, as it is dangerous. This is why R' Eliezer rules that no level of safeguarding is sufficient for it; the ox should be slaughtered so that it will not cause damage.