Sunan an-Nasa'i, Book 44, Hadith 131 (In-Book) ⓘ
It was narrated from 'Abdur-Rahman bin Abi Bakrah that his father said: "The Messenger of Allah forbade us from selling silver for silver unless it was of equal amounts, or selling gold for gold unless it was of equal amounts. The messenger of Allah said: 'Sell gold for silver however you want, and silver for gold however you want."'
أَخْبَرَنَا مُحَمَّدُ بْنُ يَحْيَى بْنِ مُحَمَّدِ بْنِ كَثِيرٍ الْحَرَّانِيُّ، قَالَ حَدَّثَنَا أَبُو تَوْبَةَ، قَالَ حَدَّثَنَا مُعَاوِيَةُ بْنُ سَلاَّمٍ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبِي بَكْرَةَ، عَنْ أَبِيهِ، قَالَ نَهَانَا رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ نَبِيعَ الْفِضَّةَ بِالْفِضَّةِ إِلاَّ عَيْنًا بِعَيْنٍ سَوَاءً بِسَوَاءٍ وَلاَ نَبِيعَ الذَّهَبَ بِالذَّهَبِ إِلاَّ عَيْنًا بِعَيْنٍ سَوَاءً بِسَوَاءٍ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " تَبَايَعُوا الذَّهَبَ بِالْفِضَّةِ كَيْفَ شِئْتُمْ وَالْفِضَّةَ بِالذَّهَبِ كَيْفَ شِئْتُمْ " .
also referenced under: Hadith 4579 (Reference) & Volume 5, Book 44, Hadith 4583 (DarusSalam)