Reference for Niddah 144:11

איהו דלא שכיחי ביה דמים לא גזור ביה רבנן איהי דשכיחי בה דמים גזור בה רבנן

but in her case, since she does usually bleed, the Rabbis enacted a preventive measure. But<span class="x" onmousemove="('comment',' Since a preventive measure was enacted in her case on account of her tendency to bleed. ');">¹¹</span>

Jastrow