Maliks Muwatta Book 31, Hadith Number 43.

Yahya related to me from Malik from Nafi that Hakim ibn Hizam traded in food for people as Umar ibn al-Khattab had ordered him to do. Hakim re-sold the food before he had taken delivery of it. That reached Umar ibn al-Khattab and he revoked the sale and said, “Do not sell food which you have purchased until you take delivery of it.”