Mukhtasar Bukhari

Narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings of Allah be upon him) forbade that anyone should sell grain before taking possession of it (i.e., before it reaches his store, shop, warehouse, etc.). Ibn Abbas (may Allah be pleased with them both) was asked the reason for this (prohibition), so he replied: This is like selling money for money, because the grain is not delivered at that time (until it comes into his store, warehouse, or shop, etc.).