Musnad Ahmad 482:38

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: Yazid ibn Abi Habib narrated to me, from Abd al-Rahman ibn Shumasah al-Tujibi, from Uqbah ibn Amir, he said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "It is not lawful for a Muslim man to propose to a woman whom his brother has already proposed to, until he abandons it; nor should he sell over the sale of his brother, until he abandons it."