Ata bin Abi Rabah (may Allah have mercy on him) said in explanation of "al-'ariyah mu'addah" (a borrowed item is to be returned): A people accepted Islam while they had items they had borrowed from the polytheists. They said: Islam has secured for us these things of the polytheists. This reached him (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Islam does not put into your possession what is not your property. A borrowed item must be returned." So those people returned those things.
Ali (al-Daraqutni) said: This is mursal, and it cannot serve as proof.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العارية / 11476