Hakim bin Hizam (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Did I not find out (or has it not reached me) that you sell grain?" Hakim (may Allah be pleased with him) said, "Yes, O Messenger of Allah." Then the Messenger of Allah (peace and blessings be upon him) said: "Do not sell the grain at all until you have bought it and become its owner."
Hadith Referenceمسند الإمام الشافعي / كتاب البيوع / 1401
Hadith Takhrijصحيح: اخرجه النسائي البيوع، باب بيع الطعام قبل أن يستوفى 4605 - واحمد: (403/3)۔