פירוש על בבא מציעא 3:7

But didn’t Rabbenai say: In Bava Kamma (113b) in [the chapter entitled] HaGozel veMakhil.

"Find it," implies it come into one's hands: And even with this, [it states,] "your brother," and not an idolater. And you should not say [that] when [the Torah] excluded idolaters, it was when it did not come to the hands of a Jew; [that] that is when you are not obligated to be burdened by it and return it; but when it comes into his hands, he is obligated to return it, since his lost object is forbidden [for you to keep].