Mishnah Niddah 1:6: And concerning what did they say that her hour suffices? [They said this] regarding the first observation [of blood], but by the second [observation] she is impure [retroactively] from that time to that time [twenty-four hours earlier]. And if she saw her first observation as a result of compulsion [i.e. if it was caused unnaturally, as by an accident], even for the second observation her hour suffices [to reckon her impurity from that moment].

וּבַמֶּה אָמְרוּ דַּיָּהּ שְׁעָתָהּ. בִּרְאִיָּה רִאשׁוֹנָה. אֲבָל בַּשְּׁנִיָּה, מְטַמְּאָה מֵעֵת לְעֵת. וְאִם רָאֲתָה הָרִאשׁוֹנָה מֵאֹנֶס, אַף הַשְּׁנִיָּה דַּיָּהּ שְׁעָתָהּ:

Mishnah Niddah 1:7: Even though they said that her hour suffices, she should be examining [herself], except for a niddah [a female who has menstrual discharges which render her impure], or one who is sitting over pure blood [i.e. following a period of impurity after giving birth, there is a period of days during which a woman remains pure even if she sees blood]. And she should have intercourse using checking-cloths [literally: witnesses, before and after intercourse], except for one who is sitting over pure blood, or a virgin whose blood is pure. And she should examine twice [daily], in the morning and at twilight, and when she going to have relations with her husband [literally: her house]. More [restricted] than this are priestly women [for they should examine themselves] at the hour that they eat from terumah [a portion of a crop given to a priest which becomes sanctified upon separation, and can only be consumed by priests or their household, and which should not be rendered impure]. Rabbi Yehuda says: [they should examine themselves] also when they are leaving from having eaten terumah.

אַף עַל פִּי שֶׁאָמְרוּ דַּיָּהּ שְׁעָתָהּ, צְרִיכָה לִהְיוֹת בּוֹדֶקֶת, חוּץ מִן הַנִּדָּה וְהַיּוֹשֶׁבֶת עַל דַּם טֹהַר. וּמְשַׁמֶּשֶׁת בְּעִדִּים, חוּץ מִיּוֹשֶׁבֶת עַל דַּם טֹהַר, וּבְתוּלָה שֶׁדָּמֶיהָ טְהוֹרִים. וּפַעֲמַיִם צְרִיכָה לִהְיוֹת בּוֹדֶקֶת, בְּשַׁחֲרִית וּבֵין הַשְּׁמָשׁוֹת, וּבְשָׁעָה שֶׁהִיא עוֹבֶרֶת לְשַׁמֵּשׁ אֶת בֵּיתָהּ. יְתֵרָה עֲלֵיהֶן כֹּהֲנוֹת, בְּשָׁעָה שֶׁהֵן אוֹכְלוֹת בַּתְּרוּמָה. רַבִּי יְהוּדָה אוֹמֵר, אַף בִּשְׁעַת עֲבָרָתָן מִלֶּאֱכֹל בַּתְּרוּמָה: