(6) باب تَحْرِيمِ الْخِطْبَةِ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَأْذَنَ أَوْ يَتْرُكَ
Abu Huraira (Allah be pleased with him) reported Allah's Messenger (ﷺ) as saying:
The Muslim should not purchase in opposition to his brother, and he should not make the proposal of marriage on the proposal already made by his brother.
Reference: Sahih Muslim 1413 d
In-book reference: Book 16, Hadith 64
USC-MSA web (English) reference(deprecated numbering scheme): Book 8, Hadith 3292