משנה על פאה 3:1

One who thins out his olive-trees [in the seventh year]: Bet Shammai says: he cuts them down to the ground. Bet Hillel says: he may completely uproot them. They agree that if one levels his field, he can only cut them down to the ground. What is considered “thinning out”? One or two plants. And what is considered “leveling”? Three plants next to each other. This applies to his own property only, but from the property of another, even he that levels may uproot.

[A loan secured by] a prozbul is not cancelled. This was one of the things enacted by Hillel the elder; for when he observed people refraining from lending to one another, and thus transgressing what is written in the Torah, “Beware, lest you harbor the base thought, [‘The seventh year, the year of remission, is approaching,’ so that you are mean to your needy kinsman and give him nothing.” Hillel enacted the prozbul.