Reference for Niddah 56:9

דתנן רבי אליעזר אומר

holding that it is necessary<span class="x" onmousemove="('comment',' If an offering is to be incurred. ');">¹⁵</span> that a person shall know<span class="x" onmousemove="('comment',' At the time he became unclean. ');">¹⁶</span>

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