Talmud for Niddah 17:18
א"ל תניתוה
examine herself since [the duty of holding an examination on the arrival of] the fixed periods is an ordinance of the Torah<span class="x" onmousemove="('comment',' Sc. a traditional halachah handed down from the time of Moses (Rashi), so that since the flow may be expected to make its appearance on the regular day, a woman who did not examine herself at such a period, must be regarded as unclean (v. infra 16a). ');"><sup>16</sup></span>
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