Reference for Niddah 106:11

ת"ר

she observed a discharge of blood she may for a period of twenty-four hours ascribe her stain to her observation;<span class="x" onmousemove="('comment',' Sc. her uncleanness does not extend retrospectively to the time the article had been washed but begins at the time the stain was found. ');">¹⁰</span>

Jastrow