Sunnah.com reference : Book 8, Hadith 12English translation : Book 8, Hadith 985Arabic reference : Book 8, Hadith 978
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 .‏‏1 ‏- صحيح .‏ رواه البخاري ( 5142 )‏ ، ومسلم ( 1412 )‏.‏