Hadith 10:64
Abu Huraira reported God’s Messenger as saying, “A man must 'not ask a woman in marriage when his brother has done so already, until he marries or gives her up." (Bukhari and Muslim.)
وَعَنْ أَبِي هُرَيْرَةَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «لَا يَخْطُبُ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَنْكِحَ أَو يتْرك»
Original Reference: Mishkat al-Masabih 3144