Reference for Eruvin 104:12
והתניא ר"א אומר אפילו אמה אחת לא יכנס כי תניא ההיא למודד דתנן ולמודד שאמרו נותנין לו אלפים אמה אפילו סוף מדתו כלה במערה:
But was it not taught: R - Eliezer ruled: Even if he was one cubit beyond his Sabbath limit he may not re-enter? - This was taught concerning a measurer,<span class="x" onmousemove="('comment',' Sc. a person who, having been overtaken by dusk on the Sabbath eve, declared the place where he stood to be his Sabbath base, and who in consequence is entitled to measure with his foot two thousand moderate steps in the direction he desires to proceed. Should the two thousand Steps, plus the four cubits to which every person is entitled as his Sabbath base, terminate even a single cubit distance from his town he may not enter it.');"><sup>21</sup></span> for we have in fact learnt: And to the measurer of whom the Rabbis have spoken a distance of two thousand cubits only is allowed<span class="x" onmousemove="('comment',' In addition to the four cubits he is allowed as his Sabbath base.');"><sup>22</sup></span>