أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏"‏ لاَ يَخْطُبْ أَحَدُكُمْ عَلَى خِطْبَةِ بَعْضٍ ‏"‏ ‏.‏

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." Translation Not Available

Book Ref: Sunan An Nisai Book 26 Hadith 3238
Web Ref:  Sunan An Nisai Vol 4 Book 26 Hadith 3240