Reference for Shevuot 50:9

הוא מותיב לה והוא מפרק לה ניכר עובר משום שבועת שוא לא ניכר עובר משום שבועת שקר

generalisation] in order to introduce an anomaly; therefore, you cannot add anything to this anomaly. - This would imply that Abaye holds that the oath [of utterance] is still in existence. But did not Abaye say: Rab admits that he who says to his neighbour, 'I swear that I know some testimony for you,' and it was found that he did not know, is exempt, because it is not applicable [negatively], 'I do not know any testimony for you'! - Abaye withdrew from that [statement]. Or, if you will, you may say,

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