Reference for Niddah 73:21

לא לטמויה לההיא טיפה במשא

it follows, that only that which causes the uncleanness of <i>zibah</i> renders void the counting of the seven days, but that which does not cause the uncleanness of <i>zibah</i> does not render void all previous counting. This is conclusive. Abaye stated: We have an accepted tradition that labour does not render void all previous counting in <i>zibah</i>; and should you find a Tanna who said that it did render the counting void, that must be R. Eliezer.<span class="x" onmousemove="('comment',' Who holds that zibah due to an accident, though it causes no zibah uncleanness, renders void all previous counting. ');">¹⁸</span>

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