(26) The Book of Marriage
كتاب النكاح
(20) Chapter: Prohibition of Proposing Marriage To A Woman When Someone Else Has Already Proposed To Her
(20) باب النَّهْىِ أَنْ يَخْطُبَ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ ‏.‏
Sunan an-Nasa'i 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 ‏"‏ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 3242
In-book reference : Book 26, Hadith 47
English translation : Vol. 4, Book 26, Hadith 3244