Babylonian Talmud, Gittin 36a

Hillel instituted the Prozbul. We have learnt elsewhere: A prozbul prevents the remission of debts [in the Sabbatical year]. This is one of the regulations made by Hillel the Elder. For he saw that people were unwilling to lend money to one another and disregarded the precept laid down in the Torah, Beware that there be not a base thought in your heart saying, etc. He therefore decided to institute the prosbul. The text of the prozbul is as follows: “I hand over to you, So-and-so, the judges in such-and-such a place, [my bonds], so that I may be able to recover any money owing to me from So-and-so at any time I shall desire”; and the prosbul was to be signed by the judges or witnesses. [AJWS translation]