Sahih Muslim Book 10, Hadith Number 3857.
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: Gold is to be paid for by gold with equal weight, like for like, and silver is to be paid for by silver with equal weight, like for like. He who made an addition to it or demanded an addition dealt in usury.