Yahya related to me from Malik from Abdullah ibn Yazid that Zaydibn Ayyash told him that he had once asked Sad ibn Abi Waqqas aboutselling white wheat for a type of good barley. Sad asked him which wasthe better and when he told him the white wheat, he forbade thetransaction. Sad said, "I heard the Messenger of Allah, may Allahbless him and grant him peace, being asked about selling dried datesfor fresh dates, and the Messenger of Allah, may Allah bless him andgrant him peace, said, 'Do the dates diminish in size when they becomedry?' When he was told that they did, he forbade that."
USC-MSA web (English) reference: Book 31, Hadith 22
Arabic reference: Book 31, Hadith 1315