Sunan an-Nasa'i, Hadith 3242 ⓘ
It was narrated from Abu Hurairah that the Prophet said: "None of you should propose marriage to a woman when someone else has already proposed to her."
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا غُنْدَرٌ، عَنْ هِشَامٍ، عَنْ مُحَمَّدٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لاَ يَخْطُبْ أَحَدُكُمْ عَلَى خِطْبَةِ أَخِيهِ " .
also referenced under: Book 26, Hadith 47 (In-Book) & Volume 4, Book 26, Hadith 3244 (DarusSalam)