Reference for Niddah 6:17

התם הלל הוא דקטעי הוא סבר

So it was also taught: Shammai said to Hillel, 'If so,<span class="x" onmousemove="('comment',' That menstrual uncleanness is to be retrospective (v. our Mishnah). ');">¹⁵</span>

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