Mukhtasar Bukhari

Narrated Abu Sa'id Al-Khudri (may Allah be pleased with him): The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not sell gold for gold except equal in weight, with neither side being more or less; and do not sell silver for silver except equal in weight, with neither side being more or less; and do not sell (gold or silver) with one side being cash and the other deferred."