Bava Kamma 114B

וקימא לן כגזלן לרבי שמעון.

And we maintain, as the Talmud concludes below, that R' Yehuda HaNasi means that the halakha that applies to a thief is like the halakha that applies to a robber according to the opinion of R' Shimon, who holds that the owner despairs of recovering his item once it is stolen by a robber.

גופא ⁠– רבי אומר, אומר אני: גנב ⁠– כגזלן. איבעיא להו: כגזלן דרבנן קאמר ⁠– ולא קני; או דלמא כגזלן דרבי שמעון קאמר, וקני?

§ The Talmud examines the matter itself. R' Yehuda HaNasi says: I say that the halakha that applies to a thief is like the halakha that applies to a robber. A dilemma was raised before the rabbis: Was R' Yehuda HaNasi saying that the halakha that applies to a thief is like the halakha that applies to a robber according to the opinion of the Rabbis, who hold that the owners do not despair of recovering their item, and a thief therefore does not acquire the items he steals? Or perhaps he was saying that the halakha that applies to a thief is like the halakha that applies to a robber according to the opinion of R' Shimon, who holds that the owners despair of recovering their property, and a thief therefore acquires the stolen items.

תא שמע: נטלו מוכסין חמורו וכו׳.

The Talmud attempts to resolve the dilemma. Come and hear a proof from the Mishnah: If customs collectors took one’s donkey and gave him a different donkey that was taken from another Jew in its stead, or if bandits took his garment and gave him a different garment that was taken from a Jew in its stead, these items are now his because the owners despaired of retrieving them when they were stolen. It is therefore apparent that despair is assumed both in the case of a robber, i.e., the customs collector, and in the case of a thief, i.e., the bandits.