Sahih Muslim
The Book of Transactions (Kitab Al-Buyu)
Book Number: 010 / Hadith Number: 3856
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.