It is narrated from Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) that he (peace and blessings be upon him) said: "Sell gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates and salt for salt, equal for equal. When these kinds differ, sell however you wish, provided the deal is hand to hand."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10512