Reference for Shevuot 80:21
רב חביבא מתני אסיפא מנה לי בידך אמר לו הן למחר אמר לו תנהו לי נתתיו לך פטור ואמר רב נחמן משביעין אותו שבועת היסת
Because it is a presumption that a man will not claim [from another] unless he has a claim upon him. - On the contrary, it is a presumption that a man will not have the effrontery [to deny] before his creditor!<span class="x" onmousemove="('comment',' And since he does deny the whole claim, he must be speaking the truth; then why an oath?');"><sup>21</sup></span>
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