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  

This hadith has been narrated by Ibn Bishr with the same chain of transmitters (but with a slight variation) that he (the Holy Prophet) inquired of them (Jews) about it (fasting on the day of 'Ashura). (Using translation from Muslim 1130b)   

عَنْ ذَلِكَ  

[Machine] So I said to him, "I refine gold and sell it by its weight, and I take a fee for my work." He said, "Do not sell gold with gold except in equal weight, and do not sell silver with silver except in equal weight. And do not take anything extra."  

فَقُلْتُ لَهُ إِنِّي أَصُوغُ الذَّهَبَ فَأَبِيعُهُ بِوَزْنِهِ وَآخُذُ لِعُمَالَةِ يَدِي أَجْرًا قَالَ لَا تَبِعِ الذَّهَبَ بِالذَّهَبِ إِلَّا وَزْنًا بِوَزْنٍ وَالْفِضَّةَ بِالْفِضَّةِ إِلَّا وَزْنًا بِوَزْنٍ وَلَا تَأْخُذْ فَضْلًا