Reference for Bava Metzia 7:6
וחזר הדין לא ראי זה כראי זה ולא ראי זה כראי זה הצד השוה שבהן שעל ידי טענה וכפירה הן באין ונשבע אף אני אביא עדים שעל ידי טענה וכפירה הם באין ונשבע
But [it is again objected]: is not 'his own mouth' more weighty in that it cannot be refuted by a denial [on the part of witnesses]? — The case of 'one witness' will prove it, in that he can be refuted [by other witnesses] and yet he obliges the defendant to take an oath. But [it is objected once more]: [The oath imposed by] one witness refers only to the part of the debt to which the witness testifies [and which the defendant denies], while [the oath that is imposed by] several witnesses refers to the remainder of the debt — [not included in the evidence and] denied by the defendant? — Again the case of 'his own mouth' will prove it.<span class="x" onmousemove="('comment',' As above, the Kal wa-homer will be inferred from the case of admission, viz., if the words of his own mouth, which do not oblige him to pay money (a fine), make it necessary for him to take an oath, how much more ought the evidence of witnesses, which obliges him to pay money, make it necessary for him to take an oath. ');"><sup>6</sup></span>