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