Sahih Muslim
The Book of Transactions (Kitab Al-Buyu)
Book Number: 010 / Hadith Number: 3857
Abu Huraira (Allah be pleased with him) reported Allah's Mess-., nger (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.