Reference for Gittin 72:1

ותקינו רבנן דתשמט זכר לשביעית ראה הלל שנמנעו העם מלהלוות זה את זה עמד והתקין פרוסבול

The Rabbis, however, ordained that it should be operative, in order to keep alive the memory of the Sabbatical year, and when Hillel saw that people refrained from lending money to one another, he decided to institute the <i>prosbul</i>.<span class="x" onmousemove="('comment',' Which therefore meant rescinding only a regulation of the Rabbis, not a precept of the Torah. ');">¹</span>

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