Sahih Muslim Book 10, Hadith Number 3853.
Chapter : He (the Holy Prophet) cursed the one who accepts interest and the one who pays it.
Ubida b. al-Samit (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, and salt by salt, like for like and equal for equal, payment being made hand to hand. If these classes differ, then sell as you wish if payment is made hand to hand.