Reference for Bava Metzia 33:18

רב כהנא אמר כשחייב מודה אי הכי מאי למימרא

but [in a case where the defendant] imposed an oath upon himself,<span class="x" onmousemove="('comment',' I.e., he offered to swear of his own accord but refused to take the oath when called upon by the plaintiff to do so in the presence of witnesses. Subsequently, however, he asserted that he did take the oath (privately), in spite of his previous refusal before witnesses. ');">¹⁷</span>

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