كتاب النكاح
16  
The Book of Marriage
(6)
Chapter: The prohibition of proposing marriage when one's brother has already proposed, unless he gives permission or gives up the idea
(6)
باب تَحْرِيمِ الْخِطْبَةِ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَأْذَنَ أَوْ يَتْرُكَ ‏‏
Sahih Muslim 1413 b

Abu Huraira (Allah be pleased with him) reported Allah's Messenger (ﷺ) as having said this:

Do not outbid in a sale in order to ensnare. No man should enter into a transaction in which his brother has already entered, and no dweller of the town should sell on behalf of the villager. And no man should make a proposal of marriage which his brother has already made and no woman should ask for the divorce of another (co-wife) in order to deprive her of what belongs to her.
وَحَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى، أَخْبَرَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، حَدَّثَنِي سَعِيدُ بْنُ الْمُسَيَّبِ، أَنَّ أَبَا هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ لاَ تَنَاجَشُوا وَلاَ يَبِعِ الْمَرْءُ عَلَى بَيْعِ أَخِيهِ وَلاَ يَبِعْ حَاضِرٌ لِبَادٍ وَلاَ يَخْطُبِ الْمَرْءُ عَلَى خِطْبَةِ أَخِيهِ وَلاَ تَسْأَلِ الْمَرْأَةُ طَلاَقَ الأُخْرَى لِتَكْتَفِئَ مَا فِي إِنَائِهَا ‏"‏ ‏.‏
Reference : Sahih Muslim 1413b
In-book reference : Book 16, Hadith 62
USC-MSA web (English) reference : Book 8, Hadith 3290
  (deprecated numbering scheme)