Sunan an-Nasa'i, Volume 4, Book 26, Hadith 3244 (DarusSalam) ⓘ
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: Hadith 3242 (Reference) & Book 26, Hadith 47 (In-Book)