Sahih Muslim Book 10, Hadith Number 3856.

Chapter : He (the Holy Prophet) cursed the one who accepts interest and the one who pays it.

Abu Huraira (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) as saying: Dates are to be paid for by dates, wheat by wheat, barley by barley, salt by salt, like for like, payment being made on the spot. He who made an addition or demanded an addition, in fact, dealt in usury except in case where their classes differ. This hadith has been narrated on the authority of Fudail b. Ghazwan with the same chain of transmitters, but he made no mention of (payment being) made on the spot.