Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Dates for dates, wheat for wheat, barley for barley and salt for salt — sell them equal for equal and hand to hand, in cash. Whoever gives more or asks for more has taken usury (riba), except when their kinds (literally, colours) are not the same."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10503