סימן קנט סעיף א
כָּל אִשָּׁה שֶׁרָאֲתָה דָם בִּימֵי טָהֳרָתָהּ, צְרִיכָה לִמְנוֹת חֲמִשָׁה יָמִים עִם יוֹם זֶה, דְּהַיְנוּ יוֹם הָרְאִיָה וְעוֹד אַרְבָּעָה יָמִים. וַאֲפִלּוּ רָאֲתָה בְּסוֹף הַיּוֹם לְאַחַר שֶׁהִתְפַּלְלוּ הַקָהָל וְגַם הִיא עַרְבִית, אוֹ קִבְּלוּ שַׁבָּת, אִם עֲדַיִן הוּא יוֹם, עוֹלֶה לָהּ יוֹם זֶה לְמִנְיָן. וּבַיוֹם הַחֲמִישִׁי לְעֵת עֶרֶב קֹדֶם בֵּין הַשְׁמָשׁוֹת תִּבְדֹּק אֶת עַצְמָּה הֵיטֵב וְתִרְחַץ לְכָל הַפָּחוֹת פָּנֶיהָ שֶׁלְּמַטָּה, וְתִּלְבַּשׁ כֻּתֹּנֶת לְבָנָה וּנְקִיָה, וְגַם שְׁאָר בְגָדֶיהָ יִהְיוּ נְקִיִים, וְזֶה נִקְרָא הֶפְסֵק טָהֳרָה. וּבַלַּיְלָה, תַּצִּיעַ עַל מִטָּתָהּ גַם כֵּן סָדִין לָבָן וְנָקִי, וְגַם הַכָּרִים וְהַכְּסָתוֹת כֻּלָם יִהְיוּ נְקִיִים, וּמִיוֹם הַמָּחֳרָת מַתְחֶלֶת לִסְפֹּר שִׁבְעָה נְקִיִים. וְאֵין חִלּוּק בֵין רְאִיָה מְרֻבָּה לִרְאִיָה מֻעֶטֶת, שֶׁאֲפִילּוּ לֹא רָאֲתָה אֶלָּא טִפָּה אַחַת, אוֹ שֶׁמָּצְאָה רַק כֶּתֶם בִּימֵי טָהָרָתָהּ, לְעוֹלָם צְרִיכָה לְהַמְתִּין חֲמִשָּׁה יָמִים. וְכֵן אֲפִלוֹ רָאֲתָה דָם כָּל חֲמֵשֶׁת הַיָמִים. אֶלָּא שֶׁפָּסְקָּה קֹדֶם בֵּין הַשְׁמָשׁוֹת, מִיָד לְאַחַר שֶׁפָּסַק הַדָּם, יְכוֹלָה לְהפְסִיק בְּטָהֳרָה.
A woman who perceived blood during the days when she is menstrually clean, must count five days including the day on which she perceived, plus another four days. Even if she perceived it at the end of the day, after the congregation, and she herself have prayed the maariv service or ushered in the Shabbos; if it is still daytime, this day is counted as one of the five.1In the event she had waited only four days and then began her seven clean days she must add another clean day. If she had already performed the ritual immersion and thereafter had marital relations with her husband, she should not perform immersion again the next day (Sidrei Taharah). Even if she had immersed but had not yet had marital relations, the Maharashaam Responsa 40 rules that she should not repeat her immersion. On the fifth day, towards evening before twilight, she should carefully examine herself, wash at least her pubic area and put on clean white undergarments. Her other garments should also be clean. This procedure is called hefsek tarahah. At night, she should spread a clean white sheet on her bed. The pillows and cushions should also be clean. The next day she should begin to count the seven clean days. It makes no difference whether she noticed much blood or little, for even if she saw only one drop, or she found only a stain during her period of purity, she must always wait another five days. Also, if she continues to bleed the entire five days, but the flow ceased before twilight (of the fifth day), immediately after the flow ceases, she may proceed with the hefsek taharah.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)