(45) باب لاَ يَخْطُبُ عَلَى خِطْبَةِ أَخِيهِ، حَتَّى يَنْكِحَ أَوْ يَدَعَ
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: Sahih al-Bukhari 5142
In-book reference: Book 67, Hadith 78
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 7, Book 62, Hadith 73