Sahih Bukhari Volume 3, Book 34, Hadith Number 350.

Narated By Abu Huraira : Allah’s Apostle forbade the selling of things by a town dweller on behalf of a desert dweller; and similarly Najsh was forbidden. And one should not urge somebody to return the goods to the seller so as to sell him his own goods; nor should one demand the hand of a girl who has already been engaged to someone else; and a woman should not try to cause some other woman to be divorced in order to take her place.