(20) باب النَّهْىِ أَنْ يَخْطُبَ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ .
It was narrated from Abu Hurairah that the Messenger of Allah said:
"None of you should propose marriage to a woman when someone else has already proposed to her, unless he marries (and he gives up the idea), or gives him permission."
Reference: Sunan an-Nasa'i 3241
In-book reference: Book 26, Hadith 46
English translation: Vol. 4, Book 26, Hadith 3243