Sahih Muslim Book 10, Hadith Number 3854.
Chapter : He (the Holy Prophet) cursed the one who accepts interest and the one who pays it.
Abu Sa’id al-Khudri (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) as saying: Gold is to be paid for by gold, silver by silver, wheat by wheat, barley by barley, dates by dates, salt by salt, like by like, payment being made hand to hand. He who made an addition to it, or asked for an addition, in fact dealt in usury. The receiver and the giver are equally guilty.