Tosefta sur Le Lévitique 19:10

[There are] four gifts [for the poor] from the vineyards: Peret (i.e., grapes that fall during harvesting, see Lev. 19:10, Peah 7:3), forgotten sheaves (Deut. 24:19), peah (Lev. 23:22), and olelet (i.e., defective clusters, see Lev. 19:10, Peah 7:4). [There are] three [gifts for the poor] from field-produce: Gleanings (Lev. 19:9), forgotten sheaves, and peah. [There are] two [gifts for the poor] from fruit trees: forgotten sheaves and peah. [And as to] all of these, they should not [be given as] a favor, and [others] can even take [gifts for the poor given as a favor] from a Jewish poor person from his hand (i.e., since it does not lawfully belong to him in the first place). And [as to] the remainder of the priestly gifts, such as the shankbone and the jawbone and the rough-stomach (see Deut. 18:3), these may be [given as] a favor. And we give [these] to every Kohen that desires [them].

What is "leket" (=gleanings)? That which falls down at the time of harvesting or of plucking (cf. Peah 4:10). Rabbi Yosei says, "There are no gleanings but that which falls down at the time of harvesting only," as it says (Lev. 19:10), "[you shall not] gather the gleanings ("leket") of your harvest."