Narrated Abu Sa'id al-Khudri (RA):

The Prophet (ﷺ) said: "Do not sell gold for gold unless it is the same amount for the same amount, and do not make one amount greater than the other. Do not sell silver for silver unless it is the same amount for the same amount, and do not make one amount greater than the other. And do not sell that (cash money) which is not present (at the time of the transaction) for that (cash money) which is present." [Agreed upon].

وَعَنْ أَبِي سَعِيدٍ اَلْخُدْرِيِّ ‏- رضى الله عنه ‏- أَنَّ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ: { لَا تَبِيعُوا اَلذَّهَبَ بِالذَّهَبِ إِلَّا مِثْلًا بِمِثْلٍ, وَلَا تُشِفُّوا [1]‏ بَعْضَهَا عَلَى بَعْضٍ, وَلَا تَبِيعُوا اَلْوَرِقَ بِالْوَرِقِ إِلَّا مِثْلًا بِمِثْلٍ, وَلَا تُشِفُّوا بَعْضَهَا عَلَى بَعْضٍ, وَلَا تَبِيعُوا مِنْهَا غَائِباً بِنَاجِزٍ } مُتَّفَقٌ عَلَيْهِ [2]‏ .‏