Reference for Bava Kamma 5:1
שׁוֹר שֶׁנָּגַח אֶת הַפָּרָה וְנִמְצָא עֻבָּרָהּ בְּצִדָּהּ, וְאֵין יָדוּעַ אִם עַד שֶׁלֹּא נְגָחָהּ יָלְדָה, אִם מִשֶּׁנְּגָחָהּ יָלְדָה, מְשַׁלֵּם חֲצִי נֶזֶק לַפָּרָה וּרְבִיעַ נֶזֶק לַוָּלָד. וְכֵן פָּרָה שֶׁנָּגְחָה אֶת הַשּׁוֹר וְנִמְצָא וְלָדָהּ בְּצִדָּהּ, וְאֵין יָדוּעַ אִם עַד שֶׁלֹּא נָגְחָה יָלְדָה, אִם מִשֶּׁנָּגְחָה יָלְדָה, מְשַׁלֵּם חֲצִי נֶזֶק מִן הַפָּרָה וּרְבִיעַ נֶזֶק מִן הַוָּלָד:
If an ox gored a [pregnant] cow, and its fetus were found [dead] at its side, and it were not known whether it gave birth before it was gored, [so that the fetus did not die because of the goring], or whether it gave birth after it was gored, [and it miscarried because of the goring], he (the owner) pays a half-nezek for the cow and a quarter-nezek for the fetus. [For a tam pays a half-nezek, and the status of this fetus is in doubt, so that they divide. Our Mishnah is in accordance with Somchos, who says that money whose ownership is in question is divided. But the sages say: This is a great principle in law — "The burden of the proof is upon him who would extract (payment) from his neighbor." And the halachah is in accordance with the sages. And even if the nizak says: "I am certain" (that your ox injured mine), and the mazik says: "Perhaps" (it did), "the burden of the proof is upon him who would extract (payment) from his neighbor."] Likewise, if a cow gored an ox, and its fetus were found at its side, and it is not known whether it gave birth before or after it gored, he pays a half-nezek from the cow and a quarter-nezek from the fetus. [He pays a half-nezek from the cow if it were found (a tam paying a half-nezek from its body), and, if it were not found, he pays a quarter-nezek from the fetus. For if it were known that it gored before it gave birth, the entire half-nezek would be paid from the fetus. For if a pregnant cow gores, it and its fetus gores. And if it gored after it gave birth, nothing would be paid from the fetus, the fetus not having gored. And a tam pays only from its body, so that now the status (of the ownership of the money) being in doubt, it is divided; and of the half-nezek that he must pay, a quarter-nezek is paid from the fetus.]