Sahih Muslim Book 010, Hadith Number 3652.
Chapter : It is invalid to sell the commodity before taking possession of it.
Abu Huraira (Allah be please with him) is reported to have said to Marwan: Have you made lawful the transactions involving interest? Thereupon Marwan said: I have not done that. Thereupon Abu Huraira (may peace be upon him) said: You have made lawful the transactions with the help of documents only, whereas Allah's Messenger (may peace be upon him) forbade the transaction of food-grains until full possession is taken of them. Marwan then addressed the people and forbade them to enter into such transactions (as are done with the help of documents). Sulaiman said: I saw the sentinels snatching (these documents) from the people.