Sahih Muslim 1589
Sahih Muslim · Book 22: The Book of Musaqah · The Prohibition of selling silver for gold to be paid at a later date
Habib reported that he heard Abu Minhal as saying:
حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ الْعَنْبَرِيُّ ، حَدَّثَنَا أَبِي ، حَدَّثَنَا شُعْبَةُ ، عَنْ حَبِيبٍ ، أَنَّهُ سَمِعَ أَبَا الْمِنْهَالِ ، يَقُولُ سَأَلْتُ الْبَرَاءَ بْنَ عَازِبٍ عَنِ الصَّرْفِ، فَقَالَ سَلْ زَيْدَ بْنَ أَرْقَمَ فَهُوَ أَعْلَمُ . فَسَأَلْتُ زَيْدًا فَقَالَ سَلِ الْبَرَاءَ فَإِنَّهُ أَعْلَمُ ثُمَّ قَالاَ نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم عَنْ بَيْعِ الْوَرِقِ بِالذَّهَبِ دَيْنًا.
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.