Reference for Bava Metzia 194:2
השוכר אומר שכורה מתה ביום שהיתה שכורה מתה בשעה שהיתה שכורה מתה והלה אומר איני יודע פטור
<b><i>GEMARA</i></b>. Hence it follows, [that if A says to B,] 'You owe me a <i>maneh</i>,' and B pleads, 'I do not know,' he is bound to pay. Shall we say that this refutes R. Nahman? For it has been taught: [If A says to B,] 'You owe me a <i>maneh</i>,' and B pleads, 'I do not know,' R. Huna and Rab Judah rule that he must pay; R. Nahman and R. Johanan say: He is not liable! — It is as R. Nahman answered [elsewhere], e.g., there is a dispute between them involving an oath; so here too, it means that there is a dispute between them involving an oath.<span class="x" onmousemove="('comment',' I.e., his plea was such that he should have taken an oath, and being unable, since he said, 'I do not know', he must pay instead, but when A claims a maneh, and B simply answers, 'I do not know', he is not thereby liable to an oath, and hence is free altogether. ');"><sup>2</sup></span>