Sahih Muslim Book 8, Hadith Number 3292.

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 saying: The Muslim should not purchase in opposition to his brother, and he should not make the proposal of marriage on the proposal already made by his brother.