Reference for Niddah 70:19

אמרו להם

And so it was also taught: If a woman who gave birth during <i>zibah</i> had counted the prescribed number of clean days but did not undergo ritual immersion and then observed a discharge. Beth Shammai gave their ruling<span class="x" onmousemove="('comment',' Lit., 'went', ');">¹⁷</span>

Jastrow