Maliks Muwatta Book 31, Hadith Number 41.

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

Yahya related to me from Malik from Abdullah ibn Dinar from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, “Someone who buys food, must not sell it until he takes possession of it.”