משנה על שמות 23:10

Bet Shammai says: [That which is] made ownerless only in regard to the poor is indeed ownerless. But Bet Hillel says: it is not ownerless unless ownership is renounced even for the rich, as in the case of the sabbatical year. [If] all of the sheaves in a field are a kav each, and one is four kavs and that one is forgotten: Bet Shammai says: it is not considered forgotten. But Bet Hillel says: it is considered forgotten.