Sahih Muslim Book 10, Hadith Number 3860.
Chapter : Accepting that which is lawful and abandoning that which is doubtful.
Habib reported that he heard Abu Minhal as saying: I asked al-Bara’ b. Azib about the exchange of (gold for silver or vice versa), whereupon he said: you better ask Zaid b. Arqam for he knows more than I. So I asked Zaid but he said: You better ask al-Bara’ for he knows more than I. Then both of them said: Allah’s Messenger (may peace be upon him) forbade the sale of silver for gold when payment is to be made in future.