Reference for Bava Metzia 7:8
ובעדים מי הוחזק כפרן והאמר רב אידי בר אבין אמר רב חסדא הכופר במלוה כשר לעדות בפקדון פסול לעדות
And the [former] argument resumes its force. [It is true that] the aspect of one case is not like the aspect of the other case; but both cases have the common characteristic that they arise through claim and denial, and therefore the defendant has to swear. So I adduce that also in the case of 'witnesses,' arising as it does through claim and denial, the defendant has to swear. But [it is again argued]: Have not the other analogous cases the common characteristic that the defendant is not presumed to be a liar, while in the case of 'witnesses' he is presumed to be a liar?<span class="x" onmousemove="('comment',' One witness cannot stamp the defendant as a liar, as it is just the word of one against that of another. But two or more witnesses are necessarily believed, and the defendant is presumed to have lied. Even if the witnesses refute only part of his statement he is not trusted any more, and should not be allowed to swear regarding the rest. ');"><sup>8</sup></span>