Kiddushin 67A

גר שנשא ממזרת, דיש קידושין ואין עבירה ⁠– הולד הולך אחר הפגום, דתניא: גר שנשא ממזרת ⁠– הולד ממזר, דברי רבי יוסי! אמר ליה: מי סברת מתניתין רבי יוסי היא? מתניתין רבי יהודה היא, דאמר: גר לא ישא ממזרת, ויש קידושין ויש עבירה ⁠– הולד הולך אחר הפגום.

there is the case of a convert who married a mamzeret, where there is a valid betrothal and there is no transgression, as they are permitted to marry each other, and yet the offspring follows the flawed lineage and is a mamzer. As it is taught in a baraita: With regard to a convert who married a mamzeret, the offspring is a mamzer. This is the statement of R' Yosei. R' Yoḥanan said to R' Shimon: Do you maintain that the Mishnah is in accordance with the opinion of R' Yosei? Not so; the Mishnah is in accordance with the opinion of R' Yehuda, who says: A convert may not marry a mamzeret, and therefore in this case there is betrothal and there is a transgression, which is why the offspring follows the flawed lineage.

וניתנייה! תנא ״כל מקום״ דסיפא, לאתויי.

The Talmud asks: But if so, let the Mishnah teach this case as one of its examples. The Talmud answers: The Mishnah taught the principle: Any case where there is betrothal and a transgression, in the latter clause, precisely to include this kind of case. There is a guiding principle of the Talmud’s interpretation of the Mishnah that a Mishnah does not include extraneous phrases. Every apparently superfluous phrase in the context of a principle serves to include or exclude a certain case from that principle. This is the basis of the discussion of the Talmud here and below.

ואיבעית אימא: לעולם רבי יוסי היא, ותנא ״איזו זו״ למעוטי.

And if you wish, say a different answer: Actually, the Mishnah is in accordance with the opinion of R' Yosei, and it taught the phrase: In which case is this applicable, to exclude cases of betrothal without a transgression other than those listed in the Mishnah. The offspring follows the lineage of the father only in those cases specified by the Mishnah.