Yusuf bin Mahak narrated from Hakim bin Hizam (may Allah be pleased with him), who said: I said: O Messenger of Allah! A man wants to buy something from me that I do not have; should I buy it (for him)? The Messenger of Allah (peace and blessings be upon him) said: "Do not sell what you do not have."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3084