Reference for Eruvin 163:18

ר"ש בן לקיש אמר כל שניעור ואינו קורא אימא אימא ס"ד גדולים נמי קרו אלא אימא כל שניעור משנתו ואינו קורא אימא אימא

May it be inferred from this ruling that no retrospective selection is valid, for if retrospective selection were valid, why should it<span class="x" onmousemove="('comment',' On the Sabbath when a townsman makes use of the 'erub.');">²⁶</span> not become known retrospectively that the man was pleased to accept the 'erub when it was yet day? - R'Ashi replied: The cases taught<span class="x" onmousemove="('comment',' In our MISHNAH:');">²⁷</span>

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