Sahih Bukhari Volume 7, Book 62, Hadith Number 73.

Narrated By Ibn ‘Umar : The Prophet decreed that one should not try to cancel a bargain already agreed upon between some other persons (by offering a bigger price). And a man should not ask for the hand of a girl who is already engaged to his Muslim brother, unless the first suitor gives her up, or allows him to ask for her hand.