MISHNA: If the ox’s owner tied it with reins to a fence or locked the gate before it in an appropriate manner, but nevertheless the ox emerged and caused damage, whether the ox is innocuous or forewarned the owner is liable, since this is not considered sufficient precaution to prevent damage; this is the statement of Rabbi Meir. Rabbi Yehuda says that if the ox is innocuous the owner is liable even if he safeguarded it appropriately, since the Torah does not limit the required safeguarding for an innocuous ox. But if the ox is forewarned, the owner is exempt from paying compensation for damage, as it is stated in the verse describing damage by a forewarned ox: “And the owner has not secured it” (Exodus 21:36), and this ox that was tied with reins or behind a locked gate was secured. Rabbi Eliezer says: An ox has no sufficient safeguarding at all other than slaughtering it with a knife; there is no degree of safeguarding that exempts the ox’s owner from liability.
GEMARA: [...] [T]his is the reason for the opinion of Rabbi Eliezer: As it is taught in a baraita that Rabbi 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 shall 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 Rabbi Eliezer rules that no level of safeguarding is sufficient for it; the ox should be slaughtered so that it will not cause damage.