Mischna zu Dewarim 24:19

These are to be considered ends of the rows:If two men begin [to gather] from the middle of the row, one facing north and the other south and they forget [some sheaves] in front of them and behind them, those left in front of them are “forgotten,” but those left behind them are not “forgotten.” If an individual begins from the end of the row and he forgets [some sheaves] in front of him and behind him, those in front of him are not “forgotten”, whereas those behind him are “forgotten,” for this comes under the category of “you shall not go back [to retrieve it].” This is the general rule: anything that can be said to fall under the law “you shall not go back” is considered “forgotten,” but that to which the principle of “you shall not go back” cannot be applied is not considered “forgotten.”