Sunan Ibn Majah, Chapter: The Chapters on Business Transactions

(the freed slave of Al-Aswad bin Sufyan), that Zaid, Abu Ayyash, the freed slave of Bani Zuhrah, told him that he asked Sad bin Abu Waqqas about buying wheat with barley. Sad said to him: "Which of them is better?" He said: "Wheat." He told him not to do that and said: "I heard the Messenger of Allah (saw) being asked about buying fresh dates with dried dates, and he said: 'Do fresh dates decrease in weight when they become dry?' They said: 'Yes.' So he told them not to do that."