Do not sell gold for gold, except like for like, and don't increase something of it upon something; and don't sell silver unless like for like, and don't increase some thing of it upon something, and do not sell for ready money something to be given later. (Using translation from Muslim 1584a)
قَالَ رَسُولُ اللهِ ﷺ لَا تَبِيعُوا الذَّهَبَ بِالذَّهَبِ إِلَّا مِثْلًا بِمِثْلٍ لَا يَشِفُّ بَعْضُهَا عَلَى بَعْضٍ وَلَا تَبِيعُوا الْوَرِقَ بِالْوَرِقِ إِلَّا مِثْلًا بِمِثْلٍ لَا يَشِفُّ بَعْضُهَا عَلَى بَعْضٍ وَلَا تَبِيعُوا غَائِبًا بِنَاجِزٍ