Reference for Ketubot 175:5

ממאן אילימא מלוה השתא אביהן שקיל בלא שבועה ואינהו בשבועה אלא ה"ק וכן היתומים מן היתומים לא יפרעו אלא בשבועה

From whom? If we say from a borrower [it may be objected;] Since their father would have received payment without an oath should they require an oath? Rather this is what was meant: And so also orphans cannot exact payment from orphans unless they first take an oath.

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