In addition to the positive mitzvah to leave the forgotten sheaf for the poor (Mitzvah #592), there is a prohibition against going back to retrieve the forgotten sheaf. This also applies to going back to retrieve a forgotten basket of fruit in an orchard. In either event, this is a lav she’nitak l’asei – a prohibition that, if violated, causes an obligation to kick in. One should not take the forgotten sheaf but if he does, he can rectify matters by turning around and giving it to the poor.
The reason for this mitzvah, as for the previous one, is to encourage us to share the good things we have received with others more needy than ourselves.
This mitzvah applies to both men and women wherever and whenever tithes are in effect. It is discussed in the Talmud in tractate Baba Metzia (11a) and in the Mishna in the sixth chapter of tractate Peah. This mitzvah is codified in the Mishneh Torah in the fifth chapter of Hilchos Matanos Aniyim. It is #214 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #10 of the 26 mitzvos that can only be performed in Israel according to the list of the Steipler Gaon.