Reference for Chullin 247:4

כי ממעט לה מד' מיהא טהור אמאי

One need not scrape off the plastering nor [see to it] that it lies the ground, but one need only cut it down to less than four handbreadths high inside.<span class="x" onmousemove="('comment',' Kel. V, 7.');"><sup>2</sup></span>