Gold is to be paid for by gold with equal weight, like for like, and silver is to be paid for by silver with equal weight, like for like. He who made an addition to it or demanded an addition dealt in usury. (Using translation from Muslim 1588c)
قَالَ رَسُولُ اللهِ ﷺ الذَّهَبُ بِالذَّهَبِ وَزْنًا بِوَزْنٍ مِثْلًا بِمِثْلٍ وَالْفِضَّةُ بِالْفِضَّةِ وَزْنًا بِوَزْنٍ مِثْلًا بِمِثْلٍ فَمَنْ زَادَ أَوِ اسْتَزَادَ فَقَدْ أَرْبَى