Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do not sell gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, or salt for salt, except equal for equal, kind for kind, hand to hand in cash. But sell gold for silver and silver for gold, wheat for barley and barley for wheat, dates for salt and salt for dates, hand to hand in cash, however you wish."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10476