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 " .
Muslim
The Book of Transactions • Hadith 12