Volume 3 · Book 37 · Hadith 490
Narrated Anas: Abdur-Rahman bin 'Auf came to us and Allah's Apostle established a bond of brotherhood between him and Sad bin Rabi'a.
— Sahih Bukhari, Volume 3, Book 37: Transferance of a Debt from One Person to Another (Al-Hawaala), Hadith #490