It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Silver should be exchanged for silver and gold for gold, equal for equal, by weight, hand to hand. Whoever adds to it or asks for more has engaged in usury.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9639