Sahih Muslim 1414
Sahih Muslim · Book 16: The Book of Marriage · The prohibition of proposing marriage when one's brother has already proposed, unless he gives permission or gives up the idea
'Uqba b. 'Amir said on the pulpit that Allah's Messenger (ﷺ) said:
وَحَدَّثَنِي أَبُو الطَّاهِرِ ، أَخْبَرَنَا عَبْدُ اللَّهِ بْنُ وَهْبٍ ، عَنِ اللَّيْثِ ، وَغَيْرِهِ، عَنْ يَزِيدَ بْنِ، أَبِي حَبِيبٍ عَنْ عَبْدِ الرَّحْمَنِ بْنِ شُمَاسَةَ ، أَنَّهُ سَمِعَ عُقْبَةَ بْنَ عَامِرٍ ، عَلَى الْمِنْبَرِ يَقُولُ إِنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " الْمُؤْمِنُ أَخُو الْمُؤْمِنِ فَلاَ يَحِلُّ لِلْمُؤْمِنِ أَنْ يَبْتَاعَ عَلَى بَيْعِ أَخِيهِ وَلاَ يَخْطُبَ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَذَرَ " .
A believer is the brother of a believer, so it is not lawful for a believer to outbid his brother, and he should not propose an engagement when his brother has thus proposed until he gives it up.