Yahya related to me from Malik from Nafi that Hakim ibn Hizamtraded in food for people as Umar ibn al-Khattab had ordered him todo. Hakim re-sold the food before he had taken delivery of it. Thatreached Umar ibn al-Khattab and he revoked the sale and said, "Do notsell food which you have purchased until you take delivery of it."
USC-MSA web (English) reference: Book 31, Hadith 43
Arabic reference: Book 31, Hadith 1335