Sahih Muslim | Book 8 | Hadith Number 3294

Narrated by 'Uqba b. 'Amir
'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.