Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Gold for gold, equal in weight, like for like, and silver for silver, equal in weight. Whoever gives more or asks for more has taken usury (riba)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10547