O men! Be conscious of your Sustainer: for, verily the violent convulsion of the Last Hour will be an awesome thing!
Al-Hajj (The Pilgrimage) Verse 1
Allah's Apostle said, "Don't sell gold for gold unless equal in weight, nor silver for silver unless equal in weight, but you could sell gold for silver or silver for gold as you like."
Sahih Bukhari: Volume 3, Book 34, Number 383