Yusuf bin Mahak al-Makki (may Allah have mercy on him) said: I used to write the accounts of orphans' expenses for so-and-so, who was their guardian. They cheated him out of a thousand dirhams, and he paid it to them. Then I came upon an equal amount of their wealth. I said: "Take the thousand that they took from you." He said: "No." For my father told me that he heard the Messenger of Allah (peace and blessings be upon him) say: "Render the trust to the one who entrusted you, and do not betray the one who betrayed you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الدعوى والبينات / 21302