Mishnayos Peah Perek 4 Mishnah 6

נכרי שקצר את שדהו וכו' – as it is written concerning gleaning and the corner of the field (Leviticus 19:9): “When you reap [the harvest of your land…],” excluding that when heathens reaped it, and concerning that which is forgotten, it is written (Deuteronomy 24:19): “When you reap the harvest in your field and overlook a sheaf in the field, [do not turn back to get it]..,” from here they said that a heathen which reaped his field and afterwards converted is exempt.

שאין השכחה אלא בשעת עמור – and at the time of carrying sheaves home, he was a convert and liable in all the commandments, but the Rabbis hold that since there is the forgotten sheaf of the Omer, and there is that which is forgotten with the standing corn, as it is written (Deuteronomy 24:19): “and overlook a sheaf in the field,” to include that which is forgotten with the standing corn, that which exists with that which is forgotten of the standing corn exists with that which is forgotten of the grain sheaf, and that convert who harvests while reaped that which was not that which was forgotten of the standing corn for at that time he was a heathen, and even though he has not converted, it is not that which is forgotten of the grain sheaf. But the Halakha is not according to Rabbi Yehuda.