Reference for Niddah 135:35

רב אמר

If a menstruant examined herself on the seventh day in the morning and found that she was unclean, and at twilight she did not ascertain her separation, and after some days she examined herself and found that she was unclean, Rab ruled: She is a certain zabah, but Levi ruled: She is a doubtful zabah. 'Rab ruled: she is a certain zabah', since she was previously found to be unclean and now also she was found to be unclean, she must be definitely unclean. 'But Levi ruled: She is a doubtful zabah', because it might be assumed that the discharge may have been discontinued in the intervening time.

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