وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، ‏.‏ أَنَّ حَكِيمَ بْنَ حِزَامٍ، ابْتَاعَ طَعَامًا أَمَرَ بِهِ عُمَرُ بْنُ الْخَطَّابِ لِلنَّاسِ فَبَاعَ حَكِيمٌ الطَّعَامَ قَبْلَ أَنْ يَسْتَوْفِيَهُ فَبَلَغَ ذَلِكَ عُمَرَ بْنَ الْخَطَّابِ فَرَدَّهُ عَلَيْهِ وَقَالَ لاَ تَبِعْ طَعَامًا ابْتَعْتَهُ حَتَّى تَسْتَوْفِيَهُ ‏.‏

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." Translation Not Available

Book Ref: Muwatta Imam Malik Book 31 Hadith 1335
Web Ref:  Muwatta Imam Malik Book 31 Hadith 43