From Sahih Bukhari (Hadith 5142):
Narrated Ibn `Umar:The Prophet (ﷺ) 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
Reference: Bukhari 5142