Do you think that you could enter paradise unless God takes cognizance of your having striven hard [in His cause], and takes cognizance of your having been patient in adversity?
Al-Imran (The Family of Imran) Verse 142
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