It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "When you buy grain, do not sell it until you have taken possession of it." Ibn Abbas (may Allah be pleased with them both) said: I consider all other sales to be like grain.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10826