Reference for Shabbat 164:2
ואמר רבה במשא דכ"ע לא פליגי דמטמאה דהא אתקש לנדה כי פליגי באבן מסמא רבי עקיבא סבר כנדה מה נדה מטמאה באבן מסמא אף ע"ז מטמאה באבן מסמא ורבנן סברי כשרץ מה שרץ לא מטמא באבן מסמא אף ע"ז לא מטמאה באבן מסמא
Rabbah also observed: As for carriage, all agree that it defiles thereby, since it is assimilated to <i>niddah</i>. They differ in respect to a stone that closes a cavity:<span class="x" onmousemove="('comment',' Rashi: a stone resting upon laths, and under it lie utensils. Tosaf.: a stone so heavy that when a niddah sits upon it her additional weight makes no difference to the utensils upon which it rests. According to both definitions, the question is whether these utensils are defiled when an idol is placed upon the stone. ');"><sup>4</sup></span> R. Akiba holds, It is like a <i>niddah</i>: just as a <i>niddah</i> defiles through a cavity-closing stone, so does an idol defile through a cavity-closing stone; while the Rabbis maintain, It is like a creeping thing [sherez]: just as a sherez does not defile through a cavity-closing stone, so does an idol not defile through a cavity-closing stone. Now, according to R. Akiba, in respect of which law is it likened to a sherez?<span class="x" onmousemove="('comment',' As it is in the verse, v.p. 393, n. 8. ');"><sup>5</sup></span>