Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) forbade "a sale combined with a loan, two sales in one sale, and selling what you do not have," and the Messenger of Allah (peace and blessings be upon him) said: "Profit is unlawful on a thing for whose loss liability has not been taken."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10880