Sahih Al Bukhari – Book of Good Manners And Form (Al-Adab)
Volumn 008, Book 073, Hadith Number 105.
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Narated By Anas : When 'Abdur-Rahman came to us, the Prophet established a bond of brotherhood between him and Sa'd bin Ar-Rabi'. Once the Prophet said, "As you (O 'Abdur-Rahman) have married, give a wedding banquet even if with one sheep."