Matar al-Warraq narrates from some of his companions that Hakim bin Hizam and Uthman bin Affan (may Allah be pleased with them both) used to bring grain from the land of Qaynuqa' to Madinah, and they would sell it by its (original) measure. The Messenger of Allah (peace and blessings be upon him) came to them and asked: "What is this?" They both said: "O Messenger of Allah! We bring goods from such-and-such a land and sell them by measure." He (peace and blessings be upon him) said: "When you buy, do not do this; rather, take possession of it, and when you sell, measure it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10699