"My eyes saw and my ears heard the Messenger of Allah. And he mentioned the prohibition of (selling) gold for gold and silver for silver, unless it is equal amounts, like for like. And do not sell it in return for something to be paid later, and do not differentiate." (Using translation from Nasāʾī 4571)
وَسَمِعَ أُذُنِي مِنْ رَسُولِ اللهِ ﷺ فَذَكَرَ النَّهْيَ عَنِ الذَّهَبِ بِالذَّهَبِ وَالْوَرِقِ بِالْوَرِقِ إِلَّا سَوَاءً بِسَوَاءٍ مِثْلًا بِمِثْلٍ وَلَا تَبِيعُوا غَائِبًا بِنَاجِزٍ وَلَا تُشِفُّوا أَحَدَهُمَا عَلَى الْآخَرِ