Moed Katan 12B

יש מהן פטור אבל אסור, ויש מהן מותר לכתחלה.

Some of the actions for which one is exempt by Torah law are nevertheless prohibited, and some of them are permitted ab initio.

רב הונא חצדו ליה חצדא במועדא. איתיביה רבה בר רב הונא לרב הונא טוחנין קמח במועד לצורך המועד, ושלא לצורך המועד ⁠— אסור. דבר שאבוד במועד ⁠— מותר לעשותו במועד, דבר שאינו אבוד במועד ⁠— אסור.

§ The Talmud relates that Rav Huna had his crop harvested for him on the intermediate days of a Festival. Rabba bar Rav Huna raised an objection to his father Rav Huna from a baraita: One may grind flour on the intermediate days of a Festival for the sake of the Festival, but if it is not for the sake of the Festival, it is prohibited. With regard to a matter that, if unattended, will result in significant loss on the Festival, it is permitted to perform labor to prevent that loss on the intermediate days of the Festival. For a matter that will not result in loss if unattended during the intermediate days of the Festival, it is prohibited to perform labor.

במה דברים אמורים ⁠— בתלושין מן הקרקע, אבל מחובר לקרקע ⁠— אפילו כולו אבוד אסור. ואם אין לו מה יאכל, קוצר ומעמר ודש וזורה ובורר וטוחן, ובלבד שלא ידוש בפרות!

In what case is this statement said? In the case of crops that are already detached from the ground, but in the case of crops that are still attached to the ground, even if it all will be lost, labor is prohibited during the intermediate days of the Festival. And if one does not have anything to eat, then he may reap and bind sheaves, and thresh, and winnow the grain in the wind, and separate the grain from the chaff, and grind the grain into flour, provided that he does not thresh with cows, the way that threshing is performed on an ordinary weekday. Why, then, did Rav Huna, who certainly had enough to eat, allow his workers to reap the field during the Festival week?