Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Gold for gold, equal for equal; silver for silver with equal weight; salt for salt with equal weight; barley for barley, equal; and wheat for wheat and dates for dates, equal. Whoever gives more or demands more has taken usury (riba). Sell gold for silver hand to hand in cash however you wish, sell dates for salt in cash, and sell barley for wheat hand to hand in cash however you wish."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10482