Mishnah Niddah 10:1: Regarding a female child whose time to see [her signs of maturity] has not yet come, if she was married, Beit Shammai say: we give her four nights [during which we assume her blood is virginal blood and not menstrual blood]. Beit Hillel say: until the wound is healed. If her time to see [her signs of maturity] has come and she is then married, Beit Shammai say: we give her the first night [during which her blood is still assumed to be virginal blood]. And Beit Hillel say: we give her four nights, until the end of Shabbat [i.e. since virgins are supposed to be wed on Wednesday]. If she sees [blood] while she is still in her father's house [i.e. before she is married], Beit Shammai say: we grant her the intercourse that is a mitzvah [i.e. the first time with her husband]; and Beit Hillel say: the entire night is hers.

תִּינוֹקֶת שֶׁלֹּא הִגִּיעַ זְמַנָּהּ לִרְאוֹת וְנִסֵּת, בֵּית שַׁמַּאי אוֹמְרִים, נוֹתְנִין לָהּ אַרְבָּעָה לֵילוֹת. וּבֵית הִלֵּל אוֹמְרִים, עַד שֶׁתִּחְיֶה הַמַּכָּה. הִגִּיעַ זְמַנָּהּ לִרְאוֹת וְנִסֵּת, בֵּית שַׁמַּאי אוֹמְרִים, נוֹתְנִין לָהּ לַיְלָה הָרִאשׁוֹן. וּבֵית הִלֵּל אוֹמְרִים, עַד מוֹצָאֵי שַׁבָּת, אַרְבָּעָה לֵילוֹת. רָאֲתָה וְעוֹדָהּ בְּבֵית אָבִיהָ, בֵּית שַׁמַּאי אוֹמְרִים, נוֹתְנִין לָהּ בְּעִילַת מִצְוָה. וּבֵית הִלֵּל אוֹמְרִים, כָּל הַלַּיְלָה כֻּלָּהּ:

Mishnah Niddah 10:2: If a niddah [a woman who has menstrual discharges which render her impure] examined herself on the seventh day [of her being a niddah] in the morning and found she was pure, and at twilight she did not [examine herself again before immersing in order to] separate [from her period of being a niddah in certain purity], and after some days she examined herself and found she was impure, she is thereby presumed to have been pure [from the night she immersed until the moment she found she was impure]. If she examined herself on the seventh day in the morning and found she was impure, and at twilight she did not [examine herself again before immersing in order to] separate [in certain purity], and after some time she examined herself and found she was pure, she is thereby assumed to have been impure [the entire time, and any pure items she handled in the interim are thereby impure]. And [in any case, when she finds herself to be impure] she renders impurity [retroactively] from that time to that time [on the previous day, i.e. twenty-four hours earlier], and from [the previous] examination to [the latest] examination. And if she has a regular cycle [when she finds she is impure], her hour suffices [to reckon the period of her impurity from that moment]. Rabbi Yehuda says: anyone who did not separate [from her period of being a niddah] in purity, from midday [on the seventh day] or later [i.e. she did not examine herself during that time to verify her purity], is thereby presumed to be impure. And the Sages say: even if she examined herself and found she was pure on the second day of her being a niddah, and at twilight she did not [examine herself again in order to] separate [from her period of being a niddah in certain purity], and after some time she examined herself and found she was impure, she is thereby presumed to have been pure [from her prior examination until the moment she found she was impure].

נִדָּה שֶׁבָּדְקָה עַצְמָהּ יוֹם שְׁבִיעִי שַׁחֲרִית וּמָצְאתָה טְהוֹרָה, וּבֵין הַשְּׁמָשׁוֹת לֹא הִפְרִישָׁה, וּלְאַחַר יָמִים בָּדְקָה וּמָצְאתָה טְמֵאָה, הֲרֵי הִיא בְחֶזְקַת טָהֳרָה. בָּדְקָה עַצְמָהּ בַּיּוֹם הַשְּׁבִיעִי בְּשַׁחֲרִית וּמָצְאתָה טְמֵאָה, וּבֵין הַשְּׁמָשׁוֹת לֹא הִפְרִישָׁה, וּלְאַחַר זְמַן בָּדְקָה וּמָצְאתָה טְהוֹרָה, הֲרֵי זוֹ בְחֶזְקַת טֻמְאָה, וּמְטַמְּאָה מֵעֵת לְעֵת וּמִפְּקִידָה לִפְקִידָה. וְאִם יֶשׁ לָהּ וֶסֶת, דַּיָּהּ שְׁעָתָהּ. רַבִּי יְהוּדָה אוֹמֵר, כֹּל שֶׁלֹּא הִפְרִישָׁה בְטָהֳרָה מִן הַמִּנְחָה וּלְמַעְלָה, הֲרֵי זוֹ בְחֶזְקַת טֻמְאָה. וַחֲכָמִים אוֹמְרִים, אֲפִלּוּ בַשְּׁנִיָּה לְנִדָּתָהּ בָּדְקָה וּמָצְאתָה טְהוֹרָה, וּבֵין הַשְּׁמָשׁוֹת לֹא הִפְרִישָׁה, וּלְאַחַר זְמַן בָּדְקָה וּמָצְאָה טְמֵאָה, הֲרֵי זוֹ בְחֶזְקַת טָהֳרָה: