Reference for Niddah 14:50

כלי שנטמא מאחוריו במשקין אחוריו טמא תוכו אזנו אוגנו ידיו טהורין

and R. Joshua ruled: They are entitled [to reckon as clean] only the first day and the seventh day,<span class="x" onmousemove="('comment',' Sc. two days only. As the cleanness of the intermediate days is a matter of doubt they must count another five days to make up the prescribed number of seven. In the case of a certain discharge on any of the days all the prescribed seven days must, of course, be counted all over again. ');">⁴⁴</span>

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