Yevamot 37B

הלכתא אהלכתא.

one halakha, i.e., that the halakha in this case is in accordance with the opinion of Rav, and another halakha, i.e., that the halakha is always decided in accordance with the opinion of R' Eliezer ben Ya’akov.

אמר אביי: מנא אמינא לה דכל ספיקא לרבי אליעזר בן יעקב ⁠— כודאי משוי ליה,

Abaye said: From where do I say that concerning anyone whose status as a mamzer is uncertain, according to the opinion of R' Eliezer ben Ya’akov they are treated equivalently to one who is definitely a mamzer?

דתניא, רבי אליעזר בן יעקב אומר: הרי שבא על נשים הרבה ואין יודע על איזו מהן בא, וכן היא שבאו עליה אנשים הרבה, ואינה יודעת מאיזה מהן קבלה ⁠— נמצא אב נושא את בתו, ואח נושא את אחותו, ונתמלא כל העולם כולו ממזרין. ועל זה נאמר: ״ומלאה הארץ זמה״.

As it is taught in a baraita that R' Eliezer ben Ya’akov says: With regard to one who had sex with and impregnated many women, but he does not know with which women he had sex, and similarly, with regard to a woman, if many men had sex with her and she became pregnant, but she does not know from which man she received the seed that caused her to become pregnant, since the identities of the parents of those children are not known, it could emerge that a father marries his daughter, and a brother marries his sister. And in this way, the entire world could become filled with mamzerim. And concerning this, it is stated: “And lest the land become full of lewdness” (Leviticus 19:29). Abaye demonstrates his claim from the fact that even though it is not certain that the children in this situation are mamzerim, nevertheless, R' Eliezer ben Ya’akov labels them as mamzerim and not as those whose status as a mamzer is uncertain.

ורבא אמר לך, הכי קאמר: ״זו מה היא״.

And Rava could have said to you: This is what the verse is saying: The word “lewdness [zima]” can be understood as an acronym of the words: Zo ma hi, meaning: What is this. It is plausible to say that R' Eliezer ben Ya’akov’s citation of this verse indicates that he regards their status to be uncertain.