Sayyiduna Abu Sa'id (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, salt for salt — sell equal for equal, in cash, hand to hand. Whoever takes more or demands more has taken usury (riba); the taker and the giver are equal in it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10487