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].

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]‏ .‏