Reference for Bava Metzia 8:4
הַמַּחֲלִיף פָּרָה בַּחֲמוֹר וְיָלְדָה, וְכֵן הַמּוֹכֵר שִׁפְחָתוֹ וְיָלְדָה, זֶה אוֹמֵר עַד שֶׁלֹּא מָכָרְתִּי, וְזֶה אוֹמֵר מִשֶּׁלָּקָחְתִּי, יַחֲלֹקוּ. הָיוּ לוֹ שְׁנֵי עֲבָדִים, אֶחָד גָּדוֹל וְאֶחָד קָטָן, וְכֵן שְׁתֵּי שָׂדוֹת, אַחַת גְּדוֹלָה וְאַחַת קְטַנָּה, הַלּוֹקֵחַ אוֹמֵר גָּדוֹל לָקַחְתִּי, וְהַלָּה אוֹמֵר אֵינִי יוֹדֵעַ, זָכָה בַגָּדוֹל. הַמּוֹכֵר אוֹמֵר קָטָן מָכָרְתִּי, וְהַלָּה אוֹמֵר אֵינִי יוֹדֵעַ, אֵין לוֹ אֶלָּא קָטָן. זֶה אוֹמֵר גָּדוֹל וְזֶה אוֹמֵר קָטָן, יִשָּׁבַע הַמּוֹכֵר שֶׁהַקָּטָן מָכָר. זֶה אוֹמֵר אֵינִי יוֹדֵעַ וְזֶה אוֹמֵר אֵינִי יוֹדֵעַ, יַחֲלֹקוּ:
If one exchanged a cow for an ass and it (the cow) gave birth; likewise, if one sold his bondswoman, and she gave birth — this one says: (She gave birth) "before I sold her"; the other: (She gave birth) "after I bought her," they divide. [For a Canaanite bondsman is acquired by money, so that when he gave the money, the bondswoman was acquired by him wherever she was. And it is not known whether he gave the money before she gave birth and the child is his (the buyer's) or after she gave birth, and the child is the (original) owner's. But a cow is not acquired by money, but by pulling. So that if he pulled it, he would know whether it had or had not given birth. This accounts for "If one exchanged, etc." For through chalifin ("exchange"), when he pulls the one (in this instance, the ass), he acquires the other (the cow) wherever it is — for which reason it is not known whether it had or had not given birth. ("they divide":) Our Mishnah is in accordance with Somchos (who holds that "money in a state of doubt is divided."). The halachah is not in accordance with him.] If one had two bondsmen: one big; the other small; likewise, two fields: one big, the other, small — The buyer says: "I bought the big one." The other says: "I do not know" — He gets the big one. If the seller says: "I sold the small one," and the other: "I do not know," he has only the small one. If one says: "A big one"; and the other: "A small one," the seller swears that he sold a small one. [i.e., This one (the buyer) says: (I gave you) money for a big bondsman; and the other (the seller): (You gave me) money for a small one. For, if a bondsman per se, the ruling is that oaths are not taken over bondsmen. What is more, what was claimed was not admitted, and what was admitted was not claimed, so that there would be no place for an oath.] If one says: "I do not know," and the other: "I do not know," they divide.