Sahih Muslim Book 10, Hadith Number 3897.
Chapter : Prohibition of taking oath in business transactions.
Abu Rafi’, the freed slave of Allah’s Messenger (may peace be upon him), said: Allah’s Messenger (may peace be upon him) took as a loan (the rest of the hadith is the same), but with this variation that he (the Holy Prophet) said: Good amongst the servants of Allah is he who is best in paying off the debt.