Sahih Muslim Book 010, Hadith Number 3617.

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 having said this: One amongst you should not enter into a transaction when another is bargaining.