صحيح البخاري Sahih Bukhari كتاب النكاح The Book of (The Wedlock)
بَابُ لاَ يَخْطُبُ عَلَى خِطْبَةِ أَخِيهِ، حَتَّى يَنْكِحَ أَوْ يَدَعَ:

(46) Chapter. None should ask for the hand of a lady who is already engaged to his brother (Muslim), but one should wait till the first suitor marries her or leaves her.

3 hadith
Hadith 5142
حَدَّثَنَا مَكِّيُّ بْنُ إِبْرَاهِيمَ ، حَدَّثَنَا ابْنُ جُرَيْجٍ ، قَالَ : سَمِعْتُ نَافِعًا يُحَدِّثُ أَنَّ ابْنَ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا ، كَانَ يَقُولُ : " نَهَى النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنْ يَبِيعَ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ ، وَلَا يَخْطُبَ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَتْرُكَ الْخَاطِبُ قَبْلَهُ ، أَوْ يَأْذَنَ لَهُ الْخَاطِبُ " .
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.
Hadith Reference Sahih Bukhari / The Book of (The Wedlock) / Hadith: 5142
Hadith Grading محدثین: أحاديث صحيح البخاريّ كلّها صحيحة
Hadith 5143
حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ ، حَدَّثَنَا اللَّيْثُ ، عَنْ جَعْفَرِ بْنِ رَبِيعَةَ ، عَنْ الْأَعْرَجِ ، قَالَ : قَالَ أَبُو هُرَيْرَةَ يَأْثُرُ : عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ، قَالَ : " إِيَّاكُمْ وَالظَّنَّ ، فَإِنَّ الظَّنَّ أَكْذَبُ الْحَدِيثِ ، وَلَا تَجَسَّسُوا ، وَلَا تَحَسَّسُوا ، وَلَا تَبَاغَضُوا ، وَكُونُوا إِخْوَانًا .
Narrated Abu Huraira: The Prophet 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.
Hadith Reference Sahih Bukhari / The Book of (The Wedlock) / Hadith: 5143
Hadith Grading محدثین: أحاديث صحيح البخاريّ كلّها صحيحة
Hadith 5144
وَلَا يَخْطُبُ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَنْكِحَ أَوْ يَتْرُكَ " .
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."
Hadith Reference Sahih Bukhari / The Book of (The Wedlock) / Hadith: 5144
Hadith Grading محدثین: أحاديث صحيح البخاريّ كلّها صحيحة