Sahih Muslim Book 10, Hadith Number 3622.

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 that Allah’s Messenger (may peace be upon him) forbade the outbidding (against another).