Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever buys foodstuff should not sell it until he has measured it." I said to Ibn Abbas (may Allah be pleased with them both): Why? He said: Do you not see that they trade gold among themselves while the foodstuff is still lying there?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10686