Sahih Muslim 1412
Sahih Muslim · Book 21: The Book of Transactions · 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
Ibn 'Umar (Allah be pleased with them) reported Allah's Messenger (ﷺ) as saying:
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 " .
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.