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 1412 b

Ibn Umar (Allah be pleased with them) reported Allah's Apostle (ﷺ) as having said this:

A person should not enter into a transaction when his brother (had already entered into but not finalised), and he should not make proposal of marriage upon the proposal already made by his brother, until he permits it.
وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، وَمُحَمَّدُ بْنُ الْمُثَنَّى، جَمِيعًا عَنْ يَحْيَى الْقَطَّانِ، قَالَ زُهَيْرٌ حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، أَخْبَرَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ لاَ يَبِعِ الرَّجُلُ عَلَى بَيْعِ أَخِيهِ وَلاَ يَخْطُبْ عَلَى خِطْبَةِ أَخِيهِ إِلاَّ أَنْ يَأْذَنَ لَهُ ‏"‏ ‏.‏
Reference : Sahih Muslim 1412 b
In-book reference : Book 16, Hadith 58
USC-MSA web (English) reference : Book 8, Hadith 3287
  (deprecated numbering scheme)