יורה דעה קפ״הYoreh De'ah 185

Se’if 1א׳

הָאִשָּׁה שֶׁהִיא בְּחֶזְקַת טְמֵאָה, אָסוּר לוֹ לָבוֹא עָלֶיהָ עַד שֶׁתֹּאמַר לוֹ: טָבַלְתִּי. הַגָּה: וּמֵאַחַר שֶׁעָבְרוּ יָמִים שֶׁאֶפְשָׁר לָהּ לִמְנוֹת וְלִטְבֹּל, נֶאֱמֶנֶת. אֲפִלּוּ רוֹאֶה בְּגָדֶיהָ מְלֻכְלָכִים בְּדָם, נֶאֱמֶנֶת לוֹמַר, בְּשׁוּק טַבָּחִים עָבַרְתִּי, אוֹ נִתְעַסַּקְתִּי בְּצִפּוֹר וְכַדּוֹמֶה לָזֶה. (בֵּית יוֹסֵף בְּשֵׁם הָרֹא"שׁ וְרַבֵּינוּ יְרוּחָם).

A woman who is in the presumption of being unclean — it is forbidden for him to come to her until she says to him: I have immersed. Hagah [Rema]: And once days have passed in which it was possible for her to count and to immerse, she is believed. Even if he sees her garments soiled with blood, she is believed to say: I passed through the butchers' market, or: I busied myself with a bird, and the like of this. (Beit Yosef in the name of the Rosh and Rabbeinu Yerucham)

Se’if 2ב׳

וְאִם הֻחְזְקָה נִדָּה בִּשְׁכֵנוֹתֶיהָ, שֶׁרָאוּהָ לוֹבֶשֶׁת בְּגָדִים הַמְיֻחָדִים לִימֵי נִדּוּתָהּ, חֲשִׁיבָה כְּוַדַּאי טְמֵאָה.

And if she was established as a niddah (a woman in her menstrual impurity) among her neighbours, in that they saw her wearing the garments set apart for the days of her niddah state, she is reckoned as certainly unclean.

Se’if 3ג׳

אָמְרָה לְבַעֲלָהּ: טְמֵאָה אֲנִי, וְאַחַר כָּךְ אָמְרָה: טְהוֹרָה אֲנִי, אֵינָהּ נֶאֱמֶנֶת. אִם הוּא לְאַחַר כְּדֵי דִּבּוּר (בֵּית יוֹסֵף בְּשֵׁם רַבֵּנוּ יְרוּחָם). וְאִם נָתְנָה אֲמַתְלָא לִדְבָרֶיהָ, כְּגוֹן שֶׁאוֹמֶרֶת שֶׁלֹּא אָמְרָה לוֹ כֵן תְּחִלָּה, אֶלָּא מִפְּנֵי שֶׁלֹּא הָיָה בָּהּ כֹּחַ לִסְבֹּל תַּשְׁמִישׁ, אוֹ טַעֲנָה אַחֶרֶת כַּיּוֹצֵא בָּזֶה, נֶאֱמֶנֶת. הַגָּה: וּמִכָּל מָקוֹם מִי שֶּׁרוֹצֶה לְהַחְמִיר עַל עַצְמוֹ, שֶׁלֹּא לְהַאֲמִין לָהּ, מִדַּת חֲסִידוּת הוּא. (בֵּית יוֹסֵף), אֲבָל מִדִּינָא נֶאֱמֶנֶת, אֲפִלּוּ בִּשְׁתִיקָה אַחַר כָּךְ רַק שֶׁהִיא בָּאָה וְשׁוֹכֶבֶת אֵצֶל בַּעֲלָהּ וְהוּא יוֹדֵעַ וּמַכִּיר שֶׁמַּה שֶּׁאָמְרָה תְּחִלָּה: טְמֵאָה אֲנִי, עָשְׂתָה מֵחֲמַת קְטָטָה שֶׁהָיָה לוֹ עִמָּהּ, (מהרי"ו סִימָן כ"ב), וְכַדּוֹמֶה לְזֶה. אֲבָל אִם רָאוּהָ לוֹבֶשֶׁת בְּגָדִים הַמְיֻחָדִים לִימֵי נִדּוּתָהּ, וְאַחַר כָּךְ אָמְרָה: טְהוֹרָה אֲנִי, אַף עַל פִּי שֶׁנָּתְנָה אֲמַתְלָא לִדְבָרֶיהָ, אֵינָהּ נֶאֱמֶנֶת. (אָמְרָה: פְּלוֹנִי חָכָם טִהֵר לִי כֶּתֶם, וְהֶחָכָם אוֹמֵר שֶׁהִיא מְשַׁקֶּרֶת, הֶחָכָם נֶאֱמָן, וּטְמֵאָה הִיא. (בֵּית יוֹסֵף רַ"ן וְרַבֵּינוּ יְרוּחָם בְּשֵׁם הָרַמְבַּ"ן).

She said to her husband: I am unclean, and afterwards said: I am clean — she is not believed, if it is after the space of an utterance (Beit Yosef in the name of Rabbeinu Yerucham). And if she gave an amatla (an explanation that accounts for her earlier words) for her words — such as where she says that she did not say so to him at the first save because she had not the strength to bear intercourse, or another plea like this — she is believed. Hagah [Rema]: And nevertheless, one who wishes to be stringent with himself, not to believe her, it is a measure of piety. (Beit Yosef) But by law she is believed, even in silence afterwards, provided only that she comes and lies down beside her husband and he knows and recognises that what she said at the first — I am unclean — she did on account of a quarrel that he had with her, (Maharyu, siman 22), and the like of this. But if they saw her wearing the garments set apart for the days of her niddah state, and afterwards she said: I am clean — even though she gave an amatla for her words, she is not believed. She said: such-and-such a scholar declared a stain clean for me, and the scholar says that she is lying — the scholar is believed, and she is unclean. (Beit Yosef, the Ran and Rabbeinu Yerucham in the name of the Ramban)