Maliks Muwatta Book 31, Hadith Number 40.
Section : Buying on Delayed Terms and Re-Selling for Less on More Immediate Terms.
Yahya related to me from Malik from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, “Someone who buys food, must not resell it until he takes delivery of it all.”