Musnad Ahmad 27:1675

Ya'qub and Sa'd narrated to us, they said: My father narrated to us, from Muhammad ibn Ishaq, who said: And Nafi', the freed slave of Abdullah ibn Umar, narrated to me, that Abdullah ibn Umar said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, forbidding that a man should propose to a woman whom his brother has already proposed to, or that he should sell over the sale of his brother.