Messenger of Allah ﷺ said, "Do not sell gold for gold unless equivalent in weight, and do not sell less amount for greater amount or vice versa; and do not sell silver for silver unless equivalent in weight, and do not sell less amount for greater amount or vice versa and do not sell gold or silver that is not present at the moment of exchange for gold or silver that is present.  

عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ ؓ أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لاَ تَبِيعُوا الذَّهَبَ بِالذَّهَبِ إِلاَّ مِثْلاً بِمِثْلٍ وَلاَ تُشِفُّوا بَعْضَهَا عَلَى بَعْضٍ وَلاَ تَبِيعُوا الْوَرِقَ بِالْوَرِقِ إِلاَّ مِثْلاً بِمِثْلٍ وَلاَ تُشِفُّوا بَعْضَهَا عَلَى بَعْضٍ وَلاَ تَبِيعُوا مِنْهَا غَائِبًا بِنَاجِزٍ