Sayyiduna Hakim bin Hizam (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) forbade me from selling what I do not have.
(b) In the narration of Hammad: the Messenger of Allah (peace and blessings be upon him) said to him: "Do not sell what you do not have."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10855