Muwatta Malik, Book 31, Hadith 30 (USC-MSA) ⓘ

Yahya related to me from Malik from Nafi from Abu Said al-Khudri that the Messenger of Allah, may Allah bless him and grant him peace, said, "Do not sell gold for gold except like for like and do not increase one part over another part. Do not sell silver for silver, except like for like and do not increase one part over another part. Do not sell some of it which is not there for some of it which is."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ لاَ تَبِيعُوا الذَّهَبَ بِالذَّهَبِ إِلاَّ مِثْلاً بِمِثْلٍ وَلاَ تُشِفُّوا بَعْضَهَا عَلَى بَعْضٍ وَلاَ تَبِيعُوا الْوَرِقَ بِالْوَرِقِ إِلاَّ مِثْلاً بِمِثْلٍ وَلاَ تُشِفُّوا بَعْضَهَا عَلَى بَعْضٍ وَلاَ تَبِيعُوا مِنْهَا شَيْئًا غَائِبًا بِنَاجِزٍ ‏" ‏ ‏.‏