"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."  

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 تَبِيعُوا مِنْهَا شَيْئًا غَائِبًا بِنَاجِزٍ  

"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."  

وَسَمِعَ أُذُنِي مِنْ رَسُولِ اللَّهِ ﷺ فَذَكَرَ النَّهْىَ عَنِ الذَّهَبِ بِالذَّهَبِ وَالْوَرِقِ بِالْوَرِقِ إِلاَّ سَوَاءً بِسَوَاءٍ مِثْلاً بِمِثْلٍ وَلاَ تَبِيعُوا غَائِبًا بِنَاجِزٍ وَلاَ تُشِفُّوا أَحَدَهُمَا عَلَى الآخَرِ  

Muawiyah sold a cup of gold or silver for more than its weight. Abu Ad-Darda' said: "I heard the Messenger of Allah forbid such transactions unless it was like for like."  

أَنَّ مُعَاوِيَةَ بَاعَ سِقَايَةً مِنْ ذَهَبٍ أَوْ وَرِقٍ بِأَكْثَرَ مِنْ وَزْنِهَا فَقَالَ أَبُو الدَّرْدَاءِ سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَنْهَى عَنْ مِثْلِ هَذَا إِلاَّ مِثْلاً بِمِثْلٍ