Reference for Bava Metzia 33:21

ואוקימנא כשחייב מודה ומשום שמא כתב ללות בניסן ולא לוה עד תשרי ואתי למטרף לקוחות מניסן ועד תשרי שלא כדין

[When this observation] was conveyed back to R. Abin, he said: I also spoke of a court case. And it was also stated so [in another place]: R. Abin said in the name of R. Elai, who said in the name of R. Johanan: If one was due [to take] an oath in a Court of Law [in regard] to [a claim of] his neighbour, and he said, 'I took the oath,' but witnesses testify that he did not take the oath, while he repeats the assertion, 'I did take the oath', [we say:] He has been found a liar in regard to this oath.

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