Sayyiduna Hakim bin Hizam (may Allah be pleased with him) says: I said: O Messenger of Allah, a man asks me for goods, that is, a sale, and the goods are not with me; may I sell them to him? He (peace and blessings be upon him) said: "Do not sell what is not with you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10422