Reference for Sanhedrin 17:2

וקמיפלגי בפלוגתא דרבי ישמעאל ורבנן דתנן מכות בשלשה משום רבי ישמעאל אמרו בעשרים ושלשה

Rabina said that [R. Meir and the Rabbis are dealing with a case] where one of the witnesses, [who testified to the woman's guilt,] was found afterwards to be a relative or otherwise disqualified. Their point of difference is the same as that in which R. Jose and Rabbi differ in applying the opinion of R. Akiba. For we learnt: R. Akiba says that the third witness<span class="x" onmousemove="('comment',' Deut. XIX, 15. Since the testimony of two suffices, the mention of the third seems superfluous. V. Mak. 5b. ');">²</span>

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