Mukhtasar Bukhari

Ameer-ul-Mu'mineen Umar bin Khattab (may Allah be pleased with him) narrates from the Noble Prophet (peace and blessings be upon him) that he said: "Selling gold for gold, wheat for wheat, dates for dates, and barley for barley is usury, except when it is equal for equal and hand to hand, then it is permissible. (Note: Selling 24-carat gold for 22-carat gold in equal weight is also usury.)"