The Messenger of Allah ﷺ said, "Do not sell gold for gold except in equal amounts, and do not give one a greater value over the other. Do not sell silver for silver except in equal amounts, and do not give one a greater value over the other. And do not sell any of it that is absent for that which is present."
أخبرنا قتيبة عن مالك عن نافع
أن رسول الله ﷺ قال لا تبيعوا الذهب بالذهب إلا مثلا بمثل ولا تشفوا بعضها على بعض ولا تبيعوا الورق بالورق إلا مثلا بمثل ولا تبيعوا منها شيئا غائبا بناجز
Bukhari transmitted it in the Book of Sales, in the chapter on selling silver for silver, hadith no. 2177, and Muslim transmitted it in the Book of Sharecropping, in the chapter on riba, hadith no. 75/1584, from the hadith of Malik through this route. It is also found in Muwatta (Yahya), 2/632, 633, and in Kubra, hadith no. 6162.