Musnad Ahmad 27:949

Hassan narrated to us, Ibn Lahi'ah narrated to us, Ubayd Allah ibn Abi Ja'far narrated to us, from Zayd ibn Aslam, who said: I heard a man ask Abdullah ibn Umar about the sale of bidding (al-muzayadah). Ibn Umar said: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade that any of you should sell over the sale of his brother, except for the spoils of war and inheritances."