Sahih Bukhari | Volume 3 | Hadith Number 385

Narrated by Abu Said Al-Khudri
Allah's Apostle said, "Do not sell gold for gold unless equivalent in weight, and do not sell less amount for greater amount or vice versa; and do not sell silver for silver unless equivalent in weight, and do not sell less amount for greater amount or vice versa and do not sell gold or silver that is not present at the moment of exchange for gold or silver that is present.