The Book of Marriage (Kitab Al-Nikah)
Muslim :: Book 8 : Hadith 3294
'Uqba b. 'Amir said on the pulpit that Allah's Messenger (may peace be upon him) said: A believer is the brother of a believer, so it is not lawful for a believer to outbid his brother, and he should not propose an engagement when his brother has thus proposed until he gives it up.
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