بُلُوغُ الْمَرَامِ
Bulugh al-Maram 978
Marriage
وَعَنِ ابْنِ عُمَرَ - رَضِيَ اَللَّهُ عَنْهُمَا- قَالَ : قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ لَا يَخْطُبْ بَعْضُكُمْ عَلَى خِطْبَةِ أَخِيهِ , حَتَّى يَتْرُكَ اَلْخَاطِبُ قَبْلَهُ , أَوْ يَأْذَنَ لَهُ اَلْخَاطِبُ } مُتَّفَقٌ عَلَيْهِ , وَاللَّفْظُ لِلْبُخَارِيِّ [1] .
Narrated Ibn 'Umar (RA): Allah's Messenger (ﷺ) said: "No one of you should ask a woman in marriage when his brother has done so already, until the one who has proposed (to her) before him gives her up, or gives him permission." [Agreed upon. The wording is al-Bukhari's].
Sunnah.com reference: Book 8, Hadith 12