Sahih Muslim Book 8, Hadith Number 3290.

Chapter: Prohibition of one’s making the proposal of marriage when it has already been made by one’s brother until he permits or gives her up.

Abu Huraira (Allah be pleased with him) reported Allah’s Messenger (May peace be upon him) as having said this: Do not outbid in a sale in order to ensnare. No man should enter into a transaction in which his brother has already entered, and no dweller of the town should sell on behalf of the villager. And no man should make a proposal of marriage which his brother has already made and no woman should ask for the divorce of another (co-wife) in order to deprive her of what belongs to her.