It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not sell gold for gold unless both are equal; do not allow any deficiency or excess on either side. And do not sell silver for silver unless both are equal; do not allow any deficiency or excess on either side. And do not sell deferred payment for immediate payment."
Hadith Referenceالسنن المأثورة / باب في البيوع / 216
Hadith Takhrijصحيح البخاري، البيوع، باب بيع الفضة بالفضة، رقم : 2177، صحيح مسلم ، المساقاة، باب الربا، رقم : 1584 ۔