Chapter: None should ask for the hand of a lady who is already engaged to his brother (Muslim)
Ibn `Umar
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)
Vol. 7, Book 62, Hadith 73
Arabic Text
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.