Sunnah.com reference : Book 7, Hadith 67English translation : Book 7, Hadith 837Arabic reference : Book 7, Hadith 834
Narrated Abu Hurairah (RA):
Allah's Messenger (ﷺ) said: "Gold is to be paid for with gold, both being of equal weight and of same quantities; silver is to be paid for with silver, both being of equal weight and of same quantities. If anyone gives more or asks for more of it, it is then usury."

[Reported by Muslim].
وَعَنْ أَبِي هُرَيْرَةَ ‏- رضى الله عنه ‏- قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ اَلذَّهَبُ بِالذَّهَبِ وَزْناً بِوَزْنٍ مِثْلًا بِمِثْلٍ، وَالْفِضَّةُ بِالْفِضَّةِ وَزْناً بِوَزْنٍ مِثْلًا بِمِثْلٍ, فَمَنْ زَادَ أَوْ اِسْتَزَادَ فَهُوَ رِبًا }  رَوَاهُ مُسْلِمٌ  1‏ .‏‏1 ‏- صحيح.‏ رواه مسلم ( 1588 )‏ ( 84 )‏.‏