Reference for Eruvin 67:2
א"ר זירא גזירה משום י"ט שחל להיות אחר שבת
if one wished, [the 'erub], though one had not actually carried it, is deemed to have been carried? - R'Zera replied: This<span class="x" onmousemove="('comment',' The carrying of an 'erub to the place one wishes to acquire as his Sabbath abode.');"><sup>1</sup></span> is a preventive measure against the possibility of [not carrying it<span class="x" onmousemove="('comment',' To the required spot.');"><sup>3</sup></span> even when] a festival occurred on a Sunday.<span class="x" onmousemove="('comment',' Lit., ' (the day) after the Sabbath'. In such a case the 'erub, if it is to be effective for the festival, must be carried to the required spot on the Sabbath eve. It cannot be taken there on the Sabbath when the carrying of objects is forbidden. Consequently, had it not been instituted that an 'erub must always be carried to the required spot, one might erroneously have formed the opinion that even in the case postulated the carrying of the 'erub to the required spot is unnecessary; and this would have had the result that the 'erub could be ineffective, since in this case carrying on the Sabbath being forbidden, the principle, 'Since it might be carried etc.' is obviously inapplicable.');"><sup>4</sup></span> He pointed out to him [another] objection: If a man, intending to acquire his Sabbath abode in a public domain, deposited his 'erub in a wall<span class="x" onmousemove="('comment',' That was more than four cubits distant from the 'abode'. If it was within the four cubits the 'erub is valid in both the following cases as explained supra in the case of a tree.');"><sup>5</sup></span>