Gemara

Question (Shmuel): If a woman tells her husband that she is Nidah and later she says that she is Tehorah, what is the law?

Answer (Rav): She is believed to give an Amasla (an explanation why she lied).

Shmuel learned this from Rav 40 times. Still, he himself did not rely on it in practice.

72a (Mishnah): The following are divorced and do not receive a Kesuvah: one who Das Moshe, e.g. having Bi'ah with him when she is Nidah...

Question: If he knows that she is Nidah, he should refrain! If he does not know that she is Nidah, he should believe her that she was not a Nidah!

Answer #1: She said that Ploni ruled that the blood she saw does not make her Nidah. He asked Ploni, and found that she lied.

Answer #2: It is like Rav Yehudah taught:

(Rav Yehudah): If a woman was established to be a Nidah among her neighbors, her husband is lashed for having Bi'ah with her.

Rishonim

The Rif and Rosh bring the Gemara verbatim.

Ran (DH Omrah): Even though she could fix the problem (if she really was Nidah) by immersing, she is believed without immersing. The Yerushalmi brings the episode with Shmuel. One day, he asked to have Bi'ah with her, and she said that she is Nidah. The next day she said that she is Tehorah, and she lied yesterday because she was weak. Even though Shmuel was stringent, it seems that the Halachah follows Rav.

Korban Nesan'el (20): The Rif and Rosh bring the episode with Shmuel to permit Midas Chasidus (to be stringent) about this. One cannot say that it is foolish Chasidus, for Shmuel did so. The Gemara says 'he did not rely in it himself'. This suggests that if someone else asked him, perhaps he would permit! Even if Shmuel argues with Rav, the Halachah follows Rav in Isurim.

Rambam (Hilchos Isurei Bi'ah 4:10): If a woman tells her husband that she is Nidah and later she says that she was joking and really she is Tehorah, she is not believed. If she gives an Amasla, she is believed. E.g. when her husband asked to had Bi'ah with her, his sister or mother was in the courtyard, and she said that she is Nidah. Later she says that she is Tehorah, and she lied lest his sister or mother see them in the act. She is believed in this and all similar cases.

Magid Mishneh and Ran (ibid.): The Rashba says that if she was established to be Nidah among her neighbors, her husband is lashed for Bi'ah with her even if she gives an Amasla. E.g. she says that she wore her Nidah clothing so he will not bother her about Bi'ah, for she was not healthy. Since he is lashed, this shows that we consider her to be Nidah and do not believe her Amasla.

Hagahos Maimoniyos (6): If she says that she erred, i.e. she thought that she was Nidah but now she sees that she is not, this is a (valid) Amasla.

Poskim

Shulchan Aruch (YD 185:3): If a woman told her husband that she is Nidah, and later she says that she is Tehorah (after Toch Kedai Dibur), she is not believed. If she gives an Amasla, e.g. at first she said that she is Nidah because she did not have strength for Bi'ah or a similar excuse, she is believed.

Beis Yosef (DH Omrah): R. Yerucham says that she is believed to retract Toch Kedai Dibur.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): Sefer ha'Terumos says that if she says that she was joking, her husband must separate from her until she counts seven clean days and immerses. If she gives an Amasla and says that that she discovered that the blood was due to a wound, or she was sick and unable to bear Bi'ah, she is believed and permitted immediately.

Toras ha'Shelamim: The same applies if she told someone else that she is Nidah, just it is Tzeni'us to discuss this only with her husband.

Rema: In any case, if a man wants to be stringent on himself not to believe her, it is Midas Chasidus.

Chasam Sofer (2:YD 149 DH Emnam, cited in Pischei Teshuvah 188:2): A man may become a Chacham, even if this reduces Onah (frequency of relations). This does not uproot his marital obligations. Even if she protests, surely at the time she married she wanted this. Likewise, one may become a Parush (stringent, e.g. if a Chacham was Metaher a reddish Bedikah) even if he was not when he married. It is normal for people to become Perushim. We say 'happy are their wives of these (the Zekenim picked to help Moshe)', even though Nevi'im are Perushim more than other men. Only Tziporah disagreed, for Moshe totally separated from her.

Rema: Letter of the law she is believed even if she is silent, but she lies next to him and he understands that what she said before was because they were quarreling or similar reasons.

Source: Mahari Veil (22), cited in Darchei Moshe (1).

Shach (4): He is required to ask her and hear the Amasla from her. Mahari Veil permits only when she initially said that she feels intestinal pain and anticipates becoming Nidah, but not if she explicitly said that she is Teme'ah.

Shulchan Aruch (ibid.): If they saw her wear clothing designated for when she is Nidah and afterwards she said that she is Tehorah, even if she gives an Amasla she is not believed.

Source (Beis Yosef DH Kosav): We learn from Kesuvos 72a that if she was established to be Nidah among her neighbors, her husband is lashed for Bi'ah with her. This shows that in such a case he may not rely on her. The Gemara says so Stam, i.e. even if she gives an Amasla.

Gra (8): Presumably she gives an Amasla in order to receive a Kesuvah. The Mishnah says that she gets no Kesuvah, i.e. she is not believed.

Question (Tur): Is being established to be Nidah among her neighbors worse than saying that she is Nidah?! After saying that she is Nidah, she is believed with an Amasla!

Answer #1 (Rashba, cited in Beis Yosef DH u'Mah she'Chosav Rabeinu v'Teimah): Due to shame or Ones, a woman is prone to falsely say that she is Nidah. She would not go so far to do an act like wearing Nidah clothing.

Shach (5): She is not believed because it would have sufficed to tell her husband that she is Nidah; there was no need to wear Nidah clothing. However, in a case where telling him would not suffice, she is believed. A case occurred in which a woman miscarried three times in the eighth month. She assumed that it was due to Ayin ha'Ra. She told her husband that she will wear Nidah clothing at the beginning of pregnancy to conceal it. The Rema (Teshuvah 2) ruled that she may give an Amasla. He would permit even if she did not tell him first, for she had no alternative. Here, it is no worse than telling him that she is Nidah, like the Tur's question!

Taz (2): This woman was not even established to be Nidah! She is like one who gave a Moda'ah (told witnesses) that the Get or gift he will 'give' is due to Ones and is invalid. The same applies to one who tells witnesses why she will 'establish' herself to be Nidah. If a woman came to a city holding a baby and raised him, they would be stoned for Bi'ah together, for he is Muchzak to be her son. If she said from the beginning that he is not her son but she is raising him, there is no Chazakah, even if later she says that he is her son. Likewise, here there is no Chazakah even after she wore Nidah clothing.

Nekudas ha'Kesef: The Rema needed to explain why she is believed even if she did not tell her husband beforehand.

Answer #2 (Taz 2, citing the Maharal): One who tells her husband knows that later she will tell him the Amasla. One would not establish herself to be Nidah in front of neighbors if it was not true, for she knows that they will not hear the Amasla. We derive that we believe her Amasla only if she told only her husband that she is Nidah, but not if she said so in front of many.

Rema: If she says 'Ploni the Chacham said that my stain is Tehorah' and Ploni says that she lies, Ploni is believed and she is Teme'ah.

Source (Beis Yosef DH b'Perek): In Kesuvos 72a we say that a woman is divorced without a Kesuvah for having Bi'ah with her husband when she is Nidah. We establish this to be when she said that Ploni was Metaher her stain, and it was found that she lied. The Ramban says that the Chacham himself is believed. The Ran (32b DH Gemara) cites the Rashba and the Yerushalmi to prove that this is when two witnesses contradict her, and also the Rosh (7:9) says so. However, that is only to make her lose her Kesuvah. All agree that the Chacham himself is believed to establish her to be Nidah.

Taz (3): She relied on the Ploni, therefore he is believed alone. If he is not here, one witness is not believed to say that Ploni ruled that she is Teme'ah.

Teshuvas Maharit (1:149, cited by Gilyon ha'Shas): David was established for 15 years to be a Kohen. He read first in the Torah and gave Birkas Kohanim. He wanted to marry a divorcee; he said that he only pretended to be a Kohen to get honor. He is not believed. People would lie, but they would not do a false act (e.g. sign - Bava Basra 30b). The Ramban says that this is why a woman who wore Nidah clothing is not believed to retract.