יורה דעה שי״זYoreh De'ah 317

Se’if 1א׳

שְׁנַיִם שֶׁהִפְקִידוּ אֵצֶל אֶחָד שְׁנֵי זְכָרִים, אֶחָד בְּכוֹר וְאֶחָד פָּשׁוּט, וּמֵת אֶחָד מֵהֶם וְאֵינוֹ יוֹדֵעַ אֵיזֶה (מֵת), מַנִּיחַ הַנִּשְׁאָר בֵּינֵיהֶם, וְהוּא בֵּין שְׁנֵיהֶם, וְדִינוֹ כִּסְפֵק בְּכוֹר.

Two who deposited two males with one man, one a firstborn and one an ordinary one, and one of them died, and he does not know which (died) — he sets down the one that remains between them, and it is between the two of them, and its law is as a doubtful firstborn.

Se’if 2ב׳

מִי שֶׁהִפְקִיד בְּכוֹר אֵצֶל חֲבֵרוֹ, וְלַחֲבֵרוֹ פָּשׁוּט (פי' שֶׁאֵינוֹ בְּכוֹר), וּמֵת אֶחָד מֵהֶם, הַמּוֹצִיא מֵחֲבֵירוֹ עָלָיו הָרְאָיָה; וְכֵיוָן שֶׁהַנִּפְקָד מֻחְזָק, יַחֲזִיקֶנּוּ וְיֹאכְלֶנּוּ מִמּוּמוֹ.

One who deposited a firstborn with his fellow, and his fellow had an ordinary one (that is, one that is not a firstborn), and one of them died — the burden of proof lies on the one who seeks to take from his fellow; and since the one with whom it was deposited is in possession, he keeps hold of it and eats it on account of its blemish.

Se’if 3ג׳

רוֹעֶה כֹּהֵן שֶׁנָּתַן בְּהֵמוֹת בַּחֲצַר בַּעַל הַבַּיִת, וְיָלְדָה אַחַת מֵהֶן בְּכוֹר, וְנִתְעָרְבוּ עִם שֶׁל בַּעַל הַבַּיִת, וּמֵת אֶחָד מֵהֶם, הָוֵי לֵיהּ בַּעַל הַבַּיִת מֻחְזָק וְיֹאכְלֶנּוּ בְּמוּמוֹ.

A shepherd who is a priest, who put animals in a householder's courtyard, and one of them bore a firstborn, and they became mixed with those of the householder, and one of them died — the householder counts as the one in possession, and he eats it in its blemish.