Lending money without witnesses brings a curse upon one's self: Bava Metzia 75b
One who lends money without witnesses will complain in court, but they won't be able to help him: Bava Metzia 75b
Designating money for a loan before three parties: Gittin 13b-14a
The designation of money for a loan works because the witnesses acquire the collateral item for the lender when they sign, at the time when the Lender gets the document: Bava Metzia 13a
