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 d

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

The Muslim should not purchase in opposition to his brother, and he should not make the proposal of marriage on the proposal already made by his brother.
حَدَّثَنَا يَحْيَى بْنُ أَيُّوبَ، وَقُتَيْبَةُ، وَابْنُ، حُجْرٍ جَمِيعًا عَنْ إِسْمَاعِيلَ بْنِ جَعْفَرٍ، - قَالَ ابْنُ أَيُّوبَ حَدَّثَنَا إِسْمَاعِيلُ، - أَخْبَرَنِي الْعَلاَءُ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ لاَ يَسُمِ الْمُسْلِمُ عَلَى سَوْمِ أَخِيهِ وَلاَ يَخْطُبْ عَلَى خِطْبَتِهِ ‏"‏ ‏.‏
Reference : Sahih Muslim 1413d
In-book reference : Book 16, Hadith 64
USC-MSA web (English) reference : Book 8, Hadith 3292
  (deprecated numbering scheme)