Sahih Muslim Book 8, Hadith Number 3294.

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.

‘Uqba b. ‘Amir said on the pulpit that Allah’s Messenger (May peace be upon him) said: A believer is the brother of a believer, so it is not lawful for a believer to outbid his brother, and he should not propose an engagement when his brother has thus proposed until he gives it up.