Reference for Bava Metzia 26:5
מצא שטרי חוב ויש בהם אחריות נכסים אף על פי ששניהם מודים לא יחזיר לא לזה ולא לזה אין בהן אחריות נכסים בזמן שהלוה מודה יחזיר למלוה אין הלוה מודה לא יחזיר לא לזה ולא לזה דברי רבי מאיר
But in a case where the debtor admits [the debt] all agree that [the document] should be returned, and that we are not afraid that the debt may have been already paid and a fraudulent agreement reached [between the lender and the borrower to exact payment from the purchasers of the borrower's property]. But R. Johanan says: The difference of opinion [in our Mishnah] concerns a case where the debtor admits [his indebtedness], R. Meir being of the opinion that a document which contains no clause mortgaging [the debtor's] property does not entitle [the creditor] to exact payment from encumbered property, but it does entitle him to exact payment from unencumbered property. But in a case where the debtor does not admit [his indebtedness]<span class="x" onmousemove="('comment',' Even if he admits that the document is genuine, but contends that the debt has been paid. ');"><sup>5</sup></span>