הלכה על בבא בתרא 154:5

The text of a contract of selling an obligation: I, x, sell you, y, the contract of the debt of z, in as z is in debt to me, as he is holding such and such; and you will acquire [the obligation] and all that is liened to it, by the force of this contract and the delivery of the contract. And if the contract is not in their hands, he gives over his possession of it, by way of its attachment to four ells of land. And [about] any oral contract, they, may their memory be blessed, said (Kiddushin 48a and see Tosafot on Bava Batra 77b:1, s.v. Rav Pappa) that it can only be acquired in the presence of the three of them (the creditor, the borrower and the purchaser of the obligation). And this is from the three things that they, may their memory be blessed, said (Gittin 14a) that they are traditional laws without an explanation.