Sayyiduna Fadalah bin Ubayd (may Allah be pleased with him) said: We were with the Messenger of Allah (peace and blessings be upon him) on the day of Khaybar, buying from the Jews an uqiyah of gold for two or three dinars. He (peace and blessings be upon him) said: "Sell gold for gold only equal in weight."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10554