None should ask for the hand of a lady who is already engaged to his brother (Muslim)
باب لاَ يَخْطُبُ عَلَى خِطْبَةِ أَخِيهِ، حَتَّى يَنْكِحَ أَوْ يَدَعَ
Sahih al-Bukhari, Book of Wedlock, Marriage (Nikaah)(view book)
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.
حَدَّثَنَا مَكِّيُّ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا ابْنُ جُرَيْجٍ، قَالَ سَمِعْتُ نَافِعًا، يُحَدِّثُ أَنَّ ابْنَ عُمَرَ ـ رضى الله عنهما ـ كَانَ يَقُولُ نَهَى النَّبِيُّ ﷺ أَنْ يَبِيعَ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ، وَلاَ يَخْطُبَ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ، حَتَّى يَتْرُكَ الْخَاطِبُ قَبْلَهُ، أَوْ يَأْذَنَ لَهُ الْخَاطِبُ.
Classification
Narrated Abu Huraira:
The Prophet ﷺ said, "Beware of suspicion (about others), as suspicion is the falsest talk, and do not spy upon each other, and do not listen to the evil talk of the people about others' affairs, and do not have enmity with one another, but be brothers. And none should ask for the hand of a girl who is already engaged to his (Muslim) brother, but one should wait till the first suitor marries her or leaves her."
حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، حَدَّثَنَا اللَّيْثُ، عَنْ جَعْفَرِ بْنِ رَبِيعَةَ، عَنِ الأَعْرَجِ، قَالَ قَالَ أَبُو هُرَيْرَةَ يَأْثُرُ عَنِ النَّبِيِّ ﷺ قَالَ " إِيَّاكُمْ وَالظَّنَّ، فَإِنَّ الظَّنَّ أَكْذَبُ الْحَدِيثِ، وَلاَ تَجَسَّسُوا، وَلاَ تَحَسَّسُوا، وَلاَ تَبَاغَضُوا، وَكُونُوا إِخْوَانًا ". وَلَا يَخْطُبُ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَنْكِحَ أَوْ يَتْرُكَ