Musnad Ahmad 28:140

Yahya ibn Ghaylan narrated to us, Rashdin narrated to us, Amr ibn al-Harith narrated to me, that Tawbah ibn Namir narrated to him, that Abu Ufayr, the chief of Sari', narrated to him, that a man asked Ibn Amr ibn al-As and said: "An orphan was under my guardianship, and I gave him a slave girl as charity. Then he died, and I am his heir." Abdullah ibn Amr said to him: "I will inform you of what I heard the Messenger of Allah, peace and blessings of Allah be upon him, say. Umar ibn al-Khattab gave a horse for the sake of Allah, then he found its owner selling it. He wanted to buy it, so he asked the Messenger of Allah, peace and blessings of Allah be upon him, and he forbade him from doing so. He said: 'If you give a charity, then let it pass [do not take it back].'"