Bekhorot 20A

מחוורתא רבי ישמעאל כרבי מאיר סבירא ליה, דחייש למיעוטא.

It is clear, as was suggested initially, that R' Yishmael holds in accordance with the opinion of R' Meir, who says that one must be concerned for the minority.

רבינא אמר: אפילו תימא רבנן, כי אזלי רבנן בתר רובא ⁠— ברובא דלא תלי במעשה, אבל רובא דתלי במעשה ⁠— לא.

Ravina says: You may even say that R' Yishmael holds in accordance with the opinion of the Rabbis. When the Rabbis follow the majority, it is with regard to a majority that is not dependent upon an action but is simply the nature of reality. But in the case of a majority that is dependent upon an action, such as the pregnancy of a young goat, which depends upon whether or not it had had sex with a male, the Rabbis do not follow the majority.

תנו רבנן: עז בת שנתה ⁠— ודאי לכהן, מיכן ולהלן ⁠— ספק. רחל בת שתים ⁠— ודאי לכהן, מיכן ולהלן ⁠— ספק. פרה בת שלש ⁠— ודאי לכהן, מיכן ולהלן ⁠— ספק. חמורה ⁠— כפרה. רבי יוסי ברבי יהודה אומר: חמורה בת ארבע שנים. עד כאן דברי רבי ישמעאל.

§ A baraita states: If one purchased a goat within its first year from a non-Jew and does not know whether or not it had previously given birth, the subsequent male offspring certainly is given to the priest; from that point forward an offspring’s status as a firstborn is uncertain. If it was a ewe within its second year the offspring certainly is given to the priest; from that point forward an offspring’s status as a firstborn is uncertain. If it was a cow within its third year the offspring certainly is given to the priest; from that point forward an offspring’s status as a firstborn is uncertain. If it was a donkey it is subject to the same halakha as a cow. R' Yosei, son of R' Yehuda, says: The offspring of a donkey within its 4th year also has the status of a firstborn. Until here is the statement of R' Yishmael.