Sahih Bukhari Volume 3, Book 34, Hadith Number 385.

Narated 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.