Sahih Bukhari
Wedlock, Marriage (Nikaah)
Volume: 007 / Book Number: 062 / Hadith Number: 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.