Sahih Muslim Book 10, Hadith Number 3861.

Chapter : Accepting that which is lawful and abandoning that which is doubtful.

Abd al-Rabman b. Abia Bakra reported on the authority of his father that Allah’s Messenger (may peace be upon him) forbade the sale of gold for gold, and silver for silver except equal for equal, and commanded us to buy silver for gold as we desired and buy gold for silver as we desired. A person asked him (about the nature of payment), whereupon he said: It is to be made on the spot. This is what I heard (from Allah’s Messenger (may peace be upon him ).