Sahih Muslim Book 8, Hadith Number 3286.

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.

Ibn Umar (Allah be pleased with them) reported Allah’s Apostle (May peace be upon him) as having said this: None amongst you should outbid another in a transaction, nor should he make proposals of marriage upon the proposal made by someone else.