Reference for Niddah 73:19

אף בשלישית בודקין אותו ברביעית אין בודקין אותו

according to R. Eliezer you may well solve from here that even that which causes no uncleanness of <i>zibah</i> renders all previous counting void. What, however, [it is asked], is the solution of the problem according to the Rabbis? — Come and hear what the father of R. Abin learnt: 'What had his <i>zibah</i> caused him? Seven days.<span class="x" onmousemove="('comment',' This is explained presently. ');">¹⁶</span>

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