Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Zayd Abu Ayyash (may Allah have mercy on him) that he asked Sayyiduna Sa’d bin Abi Waqqas (may Allah be pleased with him): Is it permissible to sell sult (a type of grain) in exchange for wheat? Sa’d (may Allah be pleased with him) asked: Which of the two is better? He replied: Wheat. So he (Sa’d, may Allah be pleased with him) forbade it and said: The Messenger of Allah (peace and blessings be upon him) was asked about buying dried dates in exchange for fresh dates, so I heard the Messenger of Allah (peace and blessings be upon him) say: Do fresh dates decrease after they become dry? The people said: Yes. So the Messenger of Allah (peace and blessings be upon him) forbade that (transaction).