Yevamot 49B

ועל הסוטה ⁠— שאין הולד ממזר.

or with a sota, that the offspring is not a mamzer.

נדה, דהא תפסי בה קידושין, שנאמר: ״ותהי נדתה עליו״, אפילו בשעת נדתה תפסי בה קידושין.

With regard to a menstruating woman the offspring is not a mamzer because one’s betrothal of her takes effect, as it is stated: “And her impurity should be upon him” (Leviticus 15:24). The phrase “should be” alludes to the fact that a betrothal with her takes effect. The verse teaches that even at the time of her menstrual impurity, betrothal with her takes effect.

סוטה נמי, דהא תפסי בה קידושין.

With regard to a sota, too, the offspring is not a mamzer because one’s betrothal of her takes effect.

תניא נמי הכי: הכל מודים בבא על הנדה ועל הסוטה ועל שומרת יבם, שאין הולד ממזר.

The Talmud notes: This teaching of Abaye is also taught in a baraita: All agree with regard to one who has sex with a menstruating woman, or with a sota, or with a widow waiting for her yavam to perform levirate marriage, that the offspring is not a mamzer.

ואביי, שומרת יבם ⁠— מספקא ליה אי כרב אי כשמואל.

The Talmud explains: And Abaye did not mention the case of a widow waiting for her yavam because he is uncertain whether, if someone other than the yavam betrothed her, the halakha is in accordance with the opinion of Rav that it does not take effect or in accordance with the opinion of Shmuel that it might take effect.