Sunan an-Nasa'i, Hadith 3241 ⓘ
It was narrated from Abu Hurairah that the Messenger of Allah said: "None of you should propose marriage to a woman when someone else has already proposed to her, unless he marries (and he gives up the idea), or gives him permission."
أَخْبَرَنِي يُونُسُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا ابْنُ وَهْبٍ، قَالَ أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ أَخْبَرَنِي سَعِيدُ بْنُ الْمُسَيَّبِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لاَ يَخْطُبْ أَحَدُكُمْ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَنْكِحَ أَوْ يَتْرُكَ " .
also referenced under: Book 26, Hadith 46 (In-Book) & Volume 4, Book 26, Hadith 3243 (DarusSalam)