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From where is it derived that it is permitted to retain the lost item of a gentile? It is derived from a verse, as it is stated with regard to the mitzva of returning a lost item: "With every lost thing of your brother's" (Deuteronomy 22:3), indicating that it is only to your brother that you return a lost item, but you do not return a lost item to a gentile.
Bava Kamma 113b:8
