Sunan an-Nasa'i, Volume 4, Book 26, Hadith 3240 (DarusSalam) ⓘ
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 " .
also referenced under: Hadith 3238 (Reference) & Book 26, Hadith 43 (In-Book)