(6) باب تَحْرِيمِ الْخِطْبَةِ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَأْذَنَ أَوْ يَتْرُكَ
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.
Reference: Sahih Muslim 1412 b
In-book reference: Book 16, Hadith 58
USC-MSA web (English) reference(deprecated numbering scheme): Book 8, Hadith 3287