Wedlock, Marriage (Nikaah)

Bukhari :: Book 7 :: Volume 62 :: Hadith 73

Narrated 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.