Musnad Ahmad 27:1816

Ibn Numayr and Muhammad ibn Ubayd narrated to us, they said: Ubaydullah narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Let none of you propose to a woman to whom his brother has already proposed, and let none of you enter into a transaction where his brother has already entered into a transaction, unless he has his permission."