Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that Sayyiduna Hakim bin Hizam (may Allah be pleased with him) bought grain. Sayyiduna Umar (may Allah be pleased with him) had ordered it for the people. Sayyiduna Hakim bin Hizam (may Allah be pleased with him) sold the grain before taking possession of it. Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) heard of this, so he reversed it for him and said: "Do not sell grain that you have bought until you take possession of it." Sayyiduna Hakim bin Hizam (may Allah be pleased with him) had bought it from its owner, so he forbade him from selling it before taking possession of it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10694