Reference for Sanhedrin 61:21

פשיטא כיון דאמר הלכה כדברי חכמים ממילא ידענא דאין הלכה כרשב"ג

HAVE TOLD HIM: 'ALL THE PROOFS WHICH YOU MAY HAVE YOU MUST PRODUCE WITHIN THIRTY DAYS:' IF HE DIES SO WITHIN THIRTY DAYS, IT UPSETS [THE DECISION]. AFTER THIRTY DAYS, IT DOES NOT. BUT RABBAN SIMEON B. GAMALIEL SAID: WHAT IS HE TO DO WHO DID NOT FIND [FAVOURABLE EVIDENCE] WITHIN THE THIRTY DAYS, BUT ONLY THEREAFTER?<span class="x" onmousemove="('comment',' I.e., even if he produces it after the stipulated period, the decision may be reversed. ');">¹⁹</span>

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