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

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 صلى الله عليه وسلم 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
Reference : Sahih al-Bukhari 5143, 5144
In-book reference : Book 67, Hadith 79
USC-MSA web (English) reference : Vol. 7, Book 62, Hadith 74
  (deprecated numbering scheme)