Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever buys grain, let him not sell it before taking possession of it." He said: We used to buy grain from the caravans by estimate, so the Messenger of Allah (peace and blessings be upon him) forbade us "to sell it before we moved it from its place."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10690