Bava Batra 176b/בבא בתרא קע״ו ב

1It is only a guarantor who undertakes a loan guarantee in the presence of a court who does not require an act of acquisition; this indicates that generally, a guarantor requires an act of acquisition in order to be obligated to pay.

2The Gemara concludes: And the halakha is that a guarantor who accepts responsibility for the loan at the time of the giving of the money does not require an act of acquisition; but if he accepts responsibility after the giving of the money, he requires an act of acquisition. Moreover, a guarantor who undertakes a loan guarantee in the presence of a court does not require an act of acquisition, as in return for that gratification that he experiences in that the court trusts him, he resolves to obligate himself.