The Prophet ﷺ said, "Return the trust to the one who entrusted you, and do not betray the one who betrayed you."
قال النبي ﷺ: أد الأمانة إلى من ائتمنك، ولا تخن من خانك.
It was narrated by Abu Dawud (3535), Tirmidhi (1264), Daraqutni (3/35), and Hakim (2/46), all through the route of Sharik and Qays, both from Abu Husayn, from Abu Salih, from Abu Hurairah, who related the hadith. Tirmidhi said, "Hasan gharib." Hakim said, "The hadith of Sharik from Abu Husayn is sahih according to the criterion of Muslim."I say, Sharik was weak in memory, and Qays followed him in this narration, he being Ibn Rabi, who is also daif. However, when narrators corroborate one another this strengthens the report, since neither of them was accused of dishonesty, but rather their weakness lay in poor memory.On this subject there is also what was reported from Yusuf bin Mahak, who said, "A man from Quraysh and I were guardians over the property of some orphans." He said, "A certain man owed me a thousand dirhams that he had taken from me. Then a thousand dirhams belonging to him came into my hand." He said, "So I said to the Qurayshi man, 'He took a thousand dirhams from me, and now a thousand dirhams of his has come into my possession.'" He said, "The Qurayshi man said, 'My father told me that he heard the Messenger of Allah ﷺ say, "Give back what is entrusted to you to the one who entrusted it to you, and do not betray the one who has betrayed you."'"This was narrated by Ahmad (15424) from Muhammad bin Abu Adi, from Humayd, from a man from the people of Makkah called Yusuf, who related it.It was also narrated by Abu Dawud (3534) from Abu Kamil, that Yazid bin Zuray narrated to them, that Humayd, meaning al-Tawil, narrated to them, from Yusuf bin Mahak al-Makki, who said, "I used to write out the expenses for the orphans of a certain man whose guardian he was. They deceived him out of a thousand dirhams, and he paid it to them. Then I recovered from their property twice that amount." He said, "I said, 'Shall I take back the thousand that they took from you?'" He said, "No." Then he said, "My father told me that he heard the Messenger of Allah ﷺ say," and he related the hadith.This narration contains an unidentified figure, the son of the companion from whom Yusuf bin Mahak narrated.It was also narrated by Daraqutni (3/35) through the route of Humayd al-Tawil, from Yusuf bin Yaqub, from a man from Quraysh, from Ubayy bin Kaab, who said, "I heard the Messenger of Allah ﷺ say," and he related the hadith.The hadith has other corroborating reports from Anas and others, though all of them contain some discussion regarding their reliability.Those who take the hadith at face value have held that it forbids anyone from taking anything that falls into his hand from the property of someone who has betrayed him. Those who do not hold to this have permitted a person to take, from the property of the one who betrayed him that has come into his possession, an amount equal to what is owed to him, interpreting the prohibition as applying only to taking more than what is rightfully his.