(20) باب النَّهْىِ أَنْ يَخْطُبَ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ .
It was narrated from Abu Hurairah that the Prophet said:
"None of you should propose marriage to a woman when someone else has already proposed to her."
Reference: Sunan an-Nasa'i 3242
In-book reference: Book 26, Hadith 47
English translation: Vol. 4, Book 26, Hadith 3244