(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 3238 
Narrated Ibn 'Umar:
It was narrated from Ibn 'Umar 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 اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ لاَ يَخْطُبْ أَحَدُكُمْ عَلَى خِطْبَةِ بَعْضٍ ‏"‏ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 3238
In-book reference : Book 26, Hadith 43
English translation : Vol. 4, Book 26, Hadith 3240