Mishnah for Bava Metzia 42:19

From when are all people permitted to take gleanings, [forgotten sheaves and peah]? After the old ones of the poor have gone. And in the case of peret and defective clusters? After the poor have gone into the vineyard and come back again. And in the case of the olive trees? After the descent of the second rainfall. Rabbi Judah said: But aren’t there those who do not harvest their olives until after the second rainfall?” Rather, once the poor man has gone out [to gather the agricultural gifts taken from olive trees] and cannot bring back with him [more than the value of] four issars.