Reference for Eruvin 121:4
ולא והתנן ולמודד שאמרו נותנין לו אלפים אמה שאפילו סוף מדתו כלה במערה
Have we not indeed? Did we not as a matter of fact learn: And to the measure<span class="x" onmousemove="('comment',' A man who measures the two thousand cubits distance from the place which he acquired as his Sabbath abode or in which he deposited his 'erub.');"><sup>12</sup></span> of whom the Rabbis have spoken a distance of two thousand cubits only is allowed even if the end of his permitted measure terminated within a cave?<span class="x" onmousemove="('comment',' Supra 52b, Mishnah infra ad fin. The interior of a cave being presumably subject to the same law as the interior of a town, R. Joshua b. Levi's ruling in respect of the latter is obviously covered by the one relating to the former. An objection against R. Idi. Aliter: Why should R. Joshua R. Levi merely repeat a Mishnah?');"><sup>13</sup></span> - His<span class="x" onmousemove="('comment',' R. Joshua b. Levis.');"><sup>14</sup></span>