Reference for Eruvin 72:1
דאי סלקא דעתך תחילת היום קונה עירוב אי אמר עירבו לי בזה אמאי לא אמר כלום
for if you should entertain the view that the validity takes effect at the beginning of the [Sabbath] day<span class="x" onmousemove="('comment',' I.e., at the end of twilight of Sabbath eve.');"><sup>1</sup></span> [the difficulty would arise:] Why 'if he said: "Prepare with this an 'erub for me" is his statement null and void'?<span class="x" onmousemove="('comment',' At the time the Sabbath begins the lagin is no longer unclean and, since its contents are proper and clean terumah, it should provide an effective 'erub. As the ruling, however, is that the 'erub is ineffective it must be concluded that the validity takes effect at the conclusion of the Sabbath eve, i.e., as explained supra, at the beginning of twilight, at which time the contents of the lag in were still tebel of the first tithe and unfit for consumption and consequently unsuitable as an 'erub.');"><sup>2</sup></span> - R'Papa retorted: It may still be maintained<span class="x" onmousemove="('comment',' Lit., 'you may even say'.');"><sup>3</sup></span>