Mishnah Niddah 1:2: How is it that her hour [of discovering menstrual blood] suffices [to reckon her impurity from that moment]? If she was sitting on a bed and was occupied with handling pure things [i.e. foods or vessels], and she left and [then] saw [blood], she is impure, and they are all pure [since her impurity is reckoned only from that moment]. Even though they said [regarding a woman who does not have a regular period]: she renders items impure from that time to that time [i.e. from twenty-fours hours earlier, retroactively], she only counts [her days of menstrual impurity] from the hour she saw [blood].

כֵּיצַד דַּיָּהּ שְׁעָתָהּ. הָיְתָה יוֹשֶׁבֶת בַּמִּטָּה וַעֲסוּקָה בְטָהֳרוֹת, וּפֵרְשָׁה וְרָאֲתָה, הִיא טְמֵאָה וְכֻלָּן טְהוֹרוֹת. אַף עַל פִּי שֶׁאָמְרוּ, מְטַמְּאָה מֵעֵת לְעֵת, אֵינָהּ מוֹנָה אֶלָּא מִשָּׁעָה שֶׁרָאָתָה:

Mishnah Niddah 1:3: Rabbi Eliezer says: There are four [types of] women whose hour [of discovering blood] suffices [to reckon their impurity from that moment]: a virgin, a pregnant woman, a nursing woman, and an elderly woman. Rabbi Yehoshua said: I only heard [this law] regarding a virgin, but the law is in accordance with Rabbi Eliezer.

רַבִּי אֱלִיעֶזֶר אוֹמֵר, אַרְבַּע נָשִׁים דַּיָּן שְׁעָתָן, בְּתוּלָה, מְעֻבֶּרֶת, מֵנִיקָה וּזְקֵנָה. אָמַר רַבִּי יְהוֹשֻׁעַ, אֲנִי לֹא שָׁמַעְתִּי אֶלָּא בְתוּלָה, אֲבָל הֲלָכָה כְּרַבִּי אֱלִיעֶזֶר: