Mishnayos Bava Kamma Perek 4 Mishnah 9

If the owner of an ox conveyed it to an unpaid bailee, or to a borrower, or to a paid bailee, or to a renter, and it caused damage while in their custody, they enter into the responsibilities and liabilities in place of the owner. Therefore, if it was forewarned the bailee pays the full cost of the damage, and if it was innocuous he pays half the cost of the damage. 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.

מְסָרוֹ לְשׁוֹמֵר חִנָּם, וּלְשׁוֹאֵל, לְנוֹשֵׂא שָׂכָר, וּלְשׂוֹכֵר, נִכְנְסוּ תַחַת הַבְּעָלִים, מוּעָד מְשַׁלֵּם נֶזֶק שָׁלֵם, וְתָם מְשַׁלֵּם חֲצִי נֶזֶק. קְשָׁרוֹ בְעָלָיו בְּמוֹסֵרָה, וְנָעַל בְּפָנָיו כָּרָאוּי, וְיָצָא וְהִזִּיק, אֶחָד תָּם וְאֶחָד מוּעָד חַיָּב, דִּבְרֵי רַבִּי מֵאִיר. רַבִּי יְהוּדָה אוֹמֵר, תָּם חַיָּב וּמוּעָד פָּטוּר, שֶׁנֶּאֱמַר (שמות כא) וְלֹא יִשְׁמְרֶנּוּ בְּעָלָיו, וְשָׁמוּר הוּא זֶה. רַבִּי אֱלִיעֶזֶר אוֹמֵר, אֵין לוֹ שְׁמִירָה אֶלָּא סַכִּין: