Zayd Abu Ayyash asked Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him) about selling fine wheat (bayda') for inferior grain (sult). Sayyiduna Sa'd (may Allah be pleased with him) said: Which of the two is better? He said: The bayda'. So Sayyiduna Sa'd (may Allah be pleased with him) forbade it and said: I heard the Messenger of Allah (peace and blessings be upon him) when he was asked about buying dried dates for fresh dates; the Prophet (peace and blessings be upon him) asked: "Do fresh dates decrease when they dry?" They said: Yes. So he (peace and blessings be upon him) forbade it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10556