Sahih Bukhari Voluume 003, Book 050, Hadith Number 884.
Narated By Abu Huraira : The Prophet said, "No town-dweller should sell for a bedouin. Do not practice Najsh (i.e. Do not offer a high price for a thing which you do not want to buy, in order to deceive the people). No Muslim should offer more for a thing already bought by his Muslim brother, nor should he demand the hand of a girl already engaged to another Muslim. A Muslim woman shall not try to bring about The divorce of her sister (i.e. another Muslim woman) in order to take her place herself."