Majma al-Zawaid — Hadith 6830

Abu Ufayr Arif bin Sari' narrates: A man asked Sayyiduna Amr bin al-As (may Allah be pleased with him), saying: "A man who was under my care - I gave him a slave girl as charity; then he died, and I am his heir." Sayyiduna Abdullah bin Amr (may Allah be pleased with him) said to him: "I will tell you what I heard from the Prophet (peace and blessings be upon him). Once Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) gave a horse in the path of Allah. Then he found that its owner had put it up for sale. Sayyiduna Umar (may Allah be pleased with him) intended to buy it, and he asked the Prophet (peace and blessings be upon him) about it. The Prophet (peace and blessings be upon him) forbade him from doing so and said: 'When you have given something in charity, let it remain as it is.'"
Ahmad recorded it; its chain contains a narrator, Rishdin bin Sa'd, who is da'if (weak), though he has also been declared trustworthy.