(16) باب النَّهْىِ عَنْ بَيْعِ الْوَرِقِ، بِالذَّهَبِ دَيْنًا
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 (ﷺ) forbade the sale of silver for gold when payment is to be made in future.
Reference: Sahih Muslim 1589 b
In-book reference: Book 22, Hadith 111
USC-MSA web (English) reference(deprecated numbering scheme): Book 10, Hadith 3860