Sifrei Devarim 282

¹(Devarim 24:19) "When you reap your harvest in the field, if you forget a sheaf in the field, do not return to take it": to exclude its being reaped by robbers, ravaged by ants, or broken by the wind or by beasts. ²" your harvest": to exclude that which appertains to the Temple and to gentiles — whence they ruled: A gentile who reaped his field and then became a convert is exempt from leket , shikchah , and peah . R. Yehudah makes him liable for shikchah ; for shikchah obtains only at the time of sheaving, (at which time he was a Jew.) ³R. Yossi Haglili says: (It is written) "When you reap your harvest in your field and you forget a sheaf": Wherever your reaping obtains, shikchah ("forgetting") obtains in the sheaves; wherever your reaping does not obtain, shikchah does not obtain in the sheaves — which is to say that if the Temple reaped (the field) and an Israelite bought it, he is exempt (from shikchah ); if a gentile reaped (the field) and an Israelite bought it, he is exempt. ⁴"in your field": excluding (from shikchah ) one who makes sheaves in his neighbor's field. These are the words of R. Meir; the sages rule it liable (for shikchah ).