Sahih Muslim Book 10, Hadith Number 3618.

Chapter : It is forbidden that one should enter into a transaction on which one’s brother has already been negotiating, or one should purchase (in opposition) to one’s brother, or one should cheat and retain milk in the under (in order to deceive the purchaser).

Ibn ‘Umar (Allah be pleased with them) reported Allah’s Messenger (may peace be upon him) as saying: A person should not enter into a transaction when his brother is already making a transaction and he should not make a proposal of marriage when his brother has already made a proposal except when lie gives permission.