Reference for Makkot 16:7

הדר אמר רבא לאו מילתא היא דאמרי קצירה דומיא דחרישה מה חרישה מצא חרוש אינו חורש אף קצירה נמי (מצא קצור אינו קוצר) ואי ס"ד מצוה מצא קצור אינו קוצר מצוה לקצור ולהביא:

He returned and said: That which I said before is not a thing (I could have given a better answer). Reaping is similar to plowing. Just as with plowing, if one found already plowed material, he does not have to plow another, so too with reaping, if he found it reaped, he does not have to reap again. And if you should think (that reaping, as brought by the verse) is a Mitzvah, if he found grain, (would he) not have to still reap? (Rather), it is a Mitzvah to reap and bring (new grain).

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