Qatadah (may Allah have mercy on him) narrates through his chain that the Messenger of Allah (peace and blessings be upon him) said: "Gold for gold, its nuggets and its coins; silver for silver, its nuggets and its coins; wheat for wheat, mudd for mudd; barley for barley, mudd for mudd; dates for dates, mudd for mudd; and salt for salt, mudd for mudd. Whoever gives more or asks for more has taken usury (riba)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10542