Language:
كتاب النكاح
67
Wedlock, Marriage (Nikaah)
(46)
Chapter: None should ask for the hand of a lady who is already engaged to his brother (Muslim)
(45)
باب لاَ يَخْطُبُ عَلَى خِطْبَةِ أَخِيهِ، حَتَّى يَنْكِحَ أَوْ يَدَعَ
Sahih al-Bukhari 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.

حَدَّثَنَا مَكِّيُّ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا ابْنُ جُرَيْجٍ، قَالَ سَمِعْتُ نَافِعًا، يُحَدِّثُ أَنَّ ابْنَ عُمَرَ ـ رضى الله عنهما ـ كَانَ يَقُولُ نَهَى النَّبِيُّ صلى الله عليه وسلم أَنْ يَبِيعَ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ، وَلاَ يَخْطُبَ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ، حَتَّى يَتْرُكَ الْخَاطِبُ قَبْلَهُ، أَوْ يَأْذَنَ لَهُ الْخَاطِبُ‏.‏
Reference : Sahih al-Bukhari 5142
In-book reference : Book 67, Hadith 78
USC-MSA web (English) reference : Vol. 7, Book 62, Hadith 73
  (deprecated numbering scheme)