Yoma 85A

ושמואל אמר לפקח עליו את הגל. כי איתמר דשמואל ⁠— ארישא איתמר: אם רוב גוים ⁠— גוי, אמר שמואל: ולענין פקוח נפש אינו כן.

And Shmuel said: This halakha of the status of a found child is with regard to removing debris from on top of him, implying that if there is a majority of non-Jews in the city where he is found, one does not violate Shabbat by removing the debris from the child to save his life. This implies that one does follow the majority in the case of saving a life. The Talmud answers: When this statement of Shmuel was stated, it was stated with regard to the first halakha. Shmuel’s intent was to be lenient, and his statement should be understood as follows: If the majority are non-Jews, he is a non-Jew. Shmuel said: But with regard to the matter of saving a life it is not so. Rather, one saves him based on the uncertainty.

אם רוב גוים ⁠— גוי, למאי הילכתא? אמר רב פפא: להאכילו נבלות. אם רוב ישראל ⁠— ישראל, למאי הילכתא? להחזיר לו אבידתו.

§ It was taught that if there is a majority of non-Jews in the city, a foundling is considered to have the status of a non-Jew. The Talmud asks: To what halakha does this statement relate? Rav Pappa said: It relates to feeding him non-kosher food. One need not protect the child from every prohibition and may even feed him non-kosher food, as though he were a non-Jew. It was further taught: If there is a majority of Jews, he is a Jew. The Talmud asks: To what halakha does this relate? The Talmud answers: It relates to returning a lost object to him. In such a case it is assumed that he is definitely a Jew. Consequently, Jews must return lost objects to him, whereas there is no obligation to return lost objects to non-Jews.