" لاَ تَبِيعُوا الذَّهَبَ بِالذَّهَبِ إِلاَّ مِثْلاً بِمِثْلٍ وَلاَ تُشِفُّوا بَعْضَهَا عَلَى بَعْضٍ وَلاَ تَبِيعُوا الْوَرِقَ بِالْوَرِقِ إِلاَّ مِثْلاً بِمِثْلٍ وَلاَ تَبِيعُوا مِنْهَا شَيْئًا غَائِبًا بِنَاجِزٍ "
It was narrated from Abu Saeed Al-Khudri that the Messenger of Allah said: "Do not sell gold for gold except like for like and do not differentiate. Do not sell silver for silver except like for like, and do not sell it in return for something to be paid later."