Ketubot 15B

מצא בה תינוק מושלך, אם רוב גוים ⁠— גוי. אם רוב ישראל ⁠— ישראל. מחצה על מחצה ⁠— ישראל.

If there is a city in which both Jews and non-Jews reside, and one found an unidentified, abandoned baby in the city, if there is a majority of non-Jews in the city the baby is deemed a non-Jew; if there is a majority of Jews in the city the baby is deemed a Jew. If half the population is non-Jew and half Jewish, the baby is deemed a Jew.

ואמר רב: לא שנו אלא להחיותו, אבל ליוחסין ⁠— לא. ושמואל אמר: לפקח עליו את הגל.

And Rav said with regard to this Mishnah: A baraita states that if there is a majority of Jews in the city the baby is deemed a Jew only with regard to sustaining him; however, with regard to lineage, e.g., marrying him to a Jewish woman, no, he is not deemed a Jew based on the majority and would require conversion. And Shmuel said: It was taught that he is deemed a Jew in order to create an opening in a heap of debris on his behalf on Shabbat, i.e., desecrating Shabbat in order to save his life. Apparently, contrary to the ruling of Rav Ḥiyya bar Ashi, Rav holds that a single majority is insufficient to deem him Jewish in matters of lineage.

אשתמיטתיה הא דאמר רב יהודה אמר רב: בקרונות של צפורי הוה מעשה.

The Talmud answers: Rav Yirmeya overlooked that which Rav Yehuda said that Rav said with regard to the Mishnah: The incident of the rape of the young girl transpired among the wagons in the marketplace of Tzippori, and there were two majorities; the majority of the inhabitants of the city and the majority of the passing contingent. Therefore, when Rav Ḥiyya bar Ashi ruled that the halakha is in accordance with the opinion of R' Yosei, i.e., that the young girl may marry a priest, it was in a case of two majorities.