Abu al-Minhal narrates: I asked Bara' bin Azib and Sayyiduna Zayd bin Arqam (may Allah be pleased with them both) about sarf (currency exchange). This one would say: Ask him, and that one would say: Ask him, for he is better than me and more knowledgeable; both of them said this. So I asked them both, and they said: "The Messenger of Allah (peace and blessings be upon him) forbade selling silver for gold on credit."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5019