Abdullah bin Ismah narrated that Hakim bin Hizam (may Allah be pleased with him) said: O Messenger of Allah! I engage in a great deal of buying and selling; so what of it is lawful for me and what is unlawful? He (peace and blessings be upon him) said: "Never sell what you have not yet taken possession of."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3094