Musnad Abu Ya'la al-Mawsili 107:29

Zuhayr narrated to us, Ya'qub ibn Ibrahim narrated to us, my father narrated to us, from Ibn Ishaq, who said: Yazid ibn Abi Habib narrated to me, from 'Abd al-Rahman ibn Shumasah al-Tujibi, from 'Uqbah ibn 'Amir, who 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. And he shall not sell over the sale of his brother, until he abandons it."