Tractate Niddah - Chapter 10 - Mishnah 2 נידה פרק י׳ משנה ב׳

If a menstruant examined herself on the seventh day in the morning and found herself to be clean, and at twilight she did not ascertain her separation [from impurity], and after some days she examined herself and found that she was unclean, she is in a presumptive state of cleanness. If she examined herself on the seventh day in the morning and found that she was unclean, and at twilight she did not ascertain her separation [impurity], and after a time she examined herself and found that she was clean, she is in a presumptive state of uncleanness. [But] she conveys uncleanness for twenty-four hours retrospectively or during the time between the last and the previous examination. But if she had a regular period, it suffices [to reckon her period of uncleanness from] the time she discovers the flow. Rabbi Judah says: Any woman who did not ascertain her separation [from impurity] is regarded as being in a presumptive state of uncleanness. But the sages say: even if she examined herself on the second day of her menstruation and found that she was clean, and at twilight she did not ascertain her separation, and after a time she examined herself and found that she was unclean, she is regarded as being in a presumptive state of cleanness.