Tosefta for Eduyot 1:1
שַׁמַּאי אוֹמֵר, כָּל הַנָּשִׁים דַּיָּן שְׁעָתָן. וְהִלֵּל אוֹמֵר, מִפְּקִידָה לִפְקִידָה, אֲפִלּוּ לְיָמִים הַרְבֵּה. וַחֲכָמִים אוֹמְרִים, לֹא כְדִבְרֵי זֶה וְלֹא כְדִבְרֵי זֶה, אֶלָּא מֵעֵת לְעֵת מְמַעֶטֶת עַל יַד מִפְּקִידָה לִפְקִידָה, וּמִפְּקִידָה לִפְקִידָה מְמַעֶטֶת עַל יַד מֵעֵת לְעֵת. כָּל אִשָּׁה שֶׁיֶּשׁ לָהּ וֶסֶת, דַּיָּהּ שְׁעָתָהּ. הַמְשַׁמֶּשֶׁת בְּעִדִּים, הֲרֵי זוֹ כִפְקִידָה, מְמַעֶטֶת עַל יַד מֵעֵת לְעֵת וְעַל יַד מִפְּקִידָה לִפְקִידָה:
Beth Shammai says: "All women — their time is sufficient." [All women who see blood, their time is sufficient to render unclean levitically clean food (taharoth) which they had touched, from the time of sighting (blood) and thenceforward. And we do not say that there was also blood before them, but the walls of the woman had prevented it from coming out, and that she was unclean even before her sighting. For all women render taharoth unclean (when they find blood) in the outer orifice, even though the blood has not come out. The reason that Shammai is not concerned that blood may have been there before her examination is that one would always be worried during intercourse that blood had issued forth and he would separate from his wife, thereby keeping the daughters of Israel from having children.] Hillel says: "From examination to examination, even for many days." [i.e., If she examined herself today and found herself clean and examined herself again at the end of the week and found herself unclean, we are concerned about what she had touched from the first examination on. Perhaps with the withdrawal of her hands (from the first examination) blood had appeared and had been prevented from coming out by the walls of the womb. And we are not concerned about suspension of cohabitation. For it is only in resepct to taharoth that we render (them) unclean from one examination to the next. But (we do) not (render her unclean) in respect to (cohabitation with) her husband. And Shammai holds that if you render her unclean in respect to taharoth, he will be uneasy and will also separate from intercourse.] And the sages say: "Neither like the one nor like the other." [Neither like Shammai, who is too lenient and does not make a fence for his words; nor like Hillel, who is overly stringent. For the womb does not hold in the blood for so long a period of time.] But "me'eth le'eth" [i.e., twenty-four hours] (sometimes) shortens al yad (i.e., after) "examination to examination," and "examination to examination" (sometimes) shortens al yad "me'eth le'eth" [i.e., Two time periods are mentioned in respect to a woman relative to rendering taharoth unclean retroactively, and the more lenient option is followed in both instances. That is, if "examination to examination" is longer than "me'eth le'eth," "me'eth le'eth" is followed and only the taharoth that she touched at this time yesterday are rendered unclean. And if "me'eth le'eth" is longer than "examination to examination," as when she examined herself in the morning and found herself clean, and in the evening and found herself unclean, only those taharoth from the first examination to the second are rendered unclean. And the halachah is in accordance with the sages. ("al yad" here is to be understood as "after," as in Nechemiah 3:8, 10, 12)]. Every woman who has a [regular] period, [established by three sightings; and she examined herself at her expected time and found herself unclean], her [expected] time is sufficient [and we do not fear that her flow had come before her time, for, certainly, the "guest" comes on time.] And if one uses cloths, this is considered an examination. [We are taught two things here, viz.: It is a mitzvah for every woman to use two examination cloths for intercourse, one before and one after. "This (the second cloth) is considered an examination," viz.] It "shortens" through "me'eth le'eth" or "examination to examination" (respectively). [For I might think that there might have been a drop of blood the size of a mustard-seed, which had been covered by semen, so that this would not constitute an examination; the Mishnah, therefore, apprises us that the ed (cloth) after intercourse is considered an examination. But the ed before intercourse is not considered an examination. For, in her haste for cohabitation, she may not be careful to insert the ed into all the crevices].