كتاب النكاح
Read the English translation first, with the original Arabic directly underneath.
It was narrated from Ibn Umar that:
the Messenger of Allah said: “A man should not propose to a woman to whom his brother has already proposed.”
لاَ يَخْطُبِ الرَّجُلُ عَلَى خِطْبَةِ أَخِيهِ