كتاب البيوع
21  
The Book of Transactions
(4)
Chapter: The prohibition of urging a buyer to cancel a purchase in order to sell him one's own goods; And urging a seller to cancel a sale already agreed upon so that one can buy the goods oneself; And the prohibition of artificially inflating prices; And the prohibition of letting milk accumulate in the udder in order to deceive the purchaser
(4)
باب تَحْرِيمِ بَيْعِ الرَّجُلِ عَلَى بَيْعِ أَخِيهِ وَسَوْمِهِ عَلَى سَوْمِهِ وَتَحْرِيمِ النَّجْشِ وَتَحْرِيمِ التَّصْرِيَةِ
Sahih Muslim 1412 f

Ibn 'Umar (Allah be pleased with them) reported Allah's Messenger (ﷺ) as saying:

A person should not enter into a transaction when his brother is already making a transaction and he should not make a proposal of marriage when his brother has already made a proposal except when he gives permission.
حَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ، وَمُحَمَّدُ بْنُ الْمُثَنَّى، - وَاللَّفْظُ لِزُهَيْرٍ - قَالاَ 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 1412f
In-book reference : Book 21, Hadith 12
USC-MSA web (English) reference : Book 10, Hadith 3618
  (deprecated numbering scheme)