Reference for Pesachim 170:8

הא גופא קשיא אמרת מן האגף ולפנים כלפנים הא אגף עצמו כלחוץ אימא סיפא מן האגף ולחוץ כלחוץ הא אגף עצמו כלפנים

AND THE THICKNESS OF THE WALL ARE AS THE INSIDE. <big><b>GEMARA: </b></big>Rab Judah said in Rab's name: And it is likewise in respect of prayer.<span class="x" onmousemove="('comment',' Certain portions of the service are recited only when there is a quorum of ten men (called minyan) . A man standing in the inside of the door-stop is counted with those inside the room, but not he who is standing outside the door-stop.');">¹⁰</span>

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