Sayyiduna Abu Umamah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Repaying a debt is obligatory, a borrowed item must be returned, an animal taken for its milk should be returned, and the one who takes responsibility is liable."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العارية / 11474
It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) borrowed coats of mail and weapons from Safwan bin Umayyah during the Battle of Hunayn. He said: O Messenger of Allah! Is a borrowed item to be returned? He (peace and blessings be upon him) said the same: "A borrowed item is to be returned."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العارية / 11475
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