Reference for Niddah 144:34

הלכה י"א

does serve as a day of observation. But 'Resh Lakish replied: The halachahs applicable to the eleventh day', i.e., neither need the eleventh be followed by one of observation nor does it serve as one of observation for the tenth.<span class="x" onmousemove="('comment',' This is the Pentateuchal law. Rabbinically, however, even the eleventh day must be followed by one of observation before the woman may be regarded as clean. ');">³¹</span>

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