Reference for Niddah 4:7
כָּל אַחַד עָשָׂר יוֹם בְּחֶזְקַת טָהֳרָה. יָשְׁבָה לָהּ וְלֹא בָדְקָה, שָׁגְגָה, נֶאֶנְסָה, הֵזִידָה וְלֹא בָדְקָה, טְהוֹרָה. הִגִּיעַ שְׁעַת וִסְתָּהּ וְלֹא בָדְקָה, הֲרֵי זוֹ טְמֵאָה. רַבִּי מֵאִיר אוֹמֵר, אִם הָיְתָה בְמַחֲבֵא וְהִגִּיעַ שְׁעַת וִסְתָּהּ, וְלֹא בָדְקָה, הֲרֵי זוֹ טְהוֹרָה, מִפְּנֵי שֶׁחֲרָדָה מְסַלֶּקֶת אֶת הַדָּמִים. אֲבָל יְמֵי הַזָּב וְהַזָּבָה וְשׁוֹמֶרֶת יוֹם כְּנֶגֶד יוֹם, הֲרֵי אֵלּוּ בְּחֶזְקַת טֻמְאָה:
Throughout all the eleven days [between each period of <i>niddah</i>] she is presumed to be pure. If [during her days of <i>niddah</i>] she sat and did not examine herself, [whether] unintentionally, under duress, or if she intentionally did not examine herself, she is pure. If the time of her regular period arrived and she did not examine herself, she is thereby impure. Rabbi Meir says: if she was in a hiding place when the time of her regular period arrived and she did not examine herself, she is thereby pure, because fear banishes blood. But [a woman] during the days of <i>zov</i> [i.e. the period of eleven days between <i>niddah</i> periods], or a <i>zavah</i> [who is supposed to be counting seven days of purity], or one who should be waiting a day [of purity] opposite each day [of impurity], they are thereby presumed to be impure [if they did not examine themselves].