Reference for Bava Metzia 67:12

תניא כוותיה דרב זביד השואל פרה מחבירו ונגנבה וקידם השואל ושילם ואח"כ נמצא הגנב משלם תשלומי כפל לשואל

Others state, R. Papa said: A borrower too, once he says 'I will pay,' the double repayment becomes his, since he could, if he wished, free himself by [the plea], 'It died on account of its work.' Thereupon R. Zebid observed to him, Thus did Abaye say: As for a borrower, [the twofold repayment is not his] unless he has actually paid. Why? — Since all the benefit [of the loan] is his, he [the lender] does not make over the double repayment to him on the strength of mere words.

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