(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." (ID)
Reference (ID): Sunan an-Nasa'i 3241 (ID)
In-book reference (ID): Book 26, Hadith 46 (ID)
English translation (ID): Vol. 4, Book 26, Hadith 3243 (ID)