(21) باب خِطْبَةِ الرَّجُلِ إِذَا تَرَكَ الْخَاطِبُ أَوْ أَذِنَ لَهُ .
'Abdullah bin 'Amr used to say:
"The Messenger of Allah forbade offering more for something that has already been bought by his brother, or for a man to propose marriage to a woman when someone else has already proposed to her, unless the previous suitor gave up the idea or gave him permission."
Reference: Sunan an-Nasa'i 3243
In-book reference: Book 26, Hadith 48
English translation: Vol. 4, Book 26, Hadith 3245