Reference for Bava Metzia 8:2
הַשּׁוֹאֵל אֶת הַפָּרָה, שְׁאָלָהּ חֲצִי הַיּוֹם וּשְׂכָרָהּ חֲצִי הַיּוֹם, שְׁאָלָהּ הַיּוֹם וּשְׂכָרָהּ לְמָחָר, שָׂכַר אַחַת וְשָׁאַל אַחַת, וָמֵתָה, הַמַּשְׁאִיל אוֹמֵר שְׁאוּלָה מֵתָה, בַּיּוֹם שֶׁהָיְתָה שְׁאוּלָה מֵתָה, בַּשָּׁעָה שֶׁהָיְתָה שְׁאוּלָה מֵתָה, וְהַלָּה אוֹמֵר אֵינִי יוֹדֵעַ, חַיָּב. הַשּׂוֹכֵר אוֹמֵר שְׂכוּרָה מֵתָה, בַּיּוֹם שֶׁהָיְתָה שְׂכוּרָה מֵתָה, בַּשָּׁעָה שֶׁהָיְתָה שְׂכוּרָה מֵתָה, וְהַלָּה אוֹמֵר אֵינִי יוֹדֵעַ, פָּטוּר. זֶה אוֹמֵר שְׁאוּלָה וְזֶה אוֹמֵר שְׂכוּרָה, יִשָּׁבַע הַשּׂוֹכֵר שֶׁשְּׂכוּרָה מֵתָה. זֶה אוֹמֵר אֵינִי יוֹדֵעַ וְזֶה אוֹמֵר אֵינִי יוֹדֵעַ, יַחֲלֹקוּ:
If one borrowed a cow: If he borrowed it for half a day and hired it for half a day; if he borrowed it for this day and hired it for the next day; if he hired one and borrowed one, and it died — The lender says: The borrowed one died; on the day it was borrowed for, it died; in the time (i.e., the part of the day) that it was borrowed for, it died [and you are liable for the accident], and the other says: I do not know [i.e., perhaps the hired one died, and I am not liable for accidents], he is liable. [This Mishnah cannot be understood as it stands, for the ruling is that if one says: "You owe me a manah and the other says: "I do not know," he swears a consuetudinal oath (shvuath heseth) that he does not know, and he is not liable. Therefore, the Gemara construes the instance as one in which he says to him: "I gave you two cows, one day to be borrowed; the other day, to be hired, and both died in the borrowing time" — and the other says: "One died in the borrowing time; the other, I do not know" — he admits part (of the claim) and is liable for an oath (that he does not owe the other part), and since he cannot swear, (not knowing), he pays. This is comparable to one's saying: "You owe me a hundred," and the other's saying: "I know (that I owe you) fifty, and I do not know (if I owe you another) fifty," in which instance he is liable for an oath, and, not being able to swear, he pays.] If the hirer says: The hired one died; on the day it was hired for, it died; in the time (i.e., the part of the day) that it was hired for, it died — and the other says: I do not know, he is not liable. If one says: The borrowed one (died), and the other: The hired one (died), the hirer swears that the hired one died. [This, too, cannot be understood as it stands, for the ruling is that if one claims wheat and the other admits barley, he is not liable — even for barley. And here, too, what was admitted was not claimed, and what was claimed was not admitted. What place is there, then, for this oath! The Gemara, therefore, construes this as an instance of an oath through gilgul ("rolling"), the one saying to the other: "Swear to me the oath of the watchers, for which you are liable, that it died a natural death," and since he takes this oath, he also takes the other, through gilgul, that the hired one died.] If one says I do not know, and the other says I do not know, they divide. [This Mishnah is in accordance with Somchos, who says: Money whose status (i.e., ownership) is in doubt is divided. This is not the halachah. The halachah is that the burden of the proof is upon him who would extract (money) from his neighbor. The claimee swears that he does not know, and he is exempt.]