Maliks Muwatta Book 31, Hadith Number 45.

Section : Buying on Delayed Terms and Re-Selling for Less on More Immediate Terms.

Yahya related to me from Malik that he had heard that a man wanted to buy food from a man in advance. The man who wanted to sell the food to him went with him to the market, and he began to show him heaps, saying, “Which one would you like me to buy for you.” The buyer said to him, “Are you selling me what you do not have?” So they came to Abdullah ibn Umarand mentioned that to him. Abdallah ibn Umar said to the buyer, “Do not buy from him what he does not have.” He said to the seller, “Do not sell what you do not have.”