Sahih Muslim Book 10, Hadith Number 3619.

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).

Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: A Muslim should not purchase (in opposition) to his brother. This hadith has been narrated on the authority of Abu Huraira through another chain of transmitters but with a slight change of words.