It is narrated from Abu Sa'id Al-Khudri (may Allah be pleased with him) that the Messenger of Allah (ﷺ) said: "Do not sell gold for gold except equal for equal, and do not increase one over the other; and do not sell silver for silver except equal for equal, hand to hand, and do not increase one over the other; and do not sell something absent for something present among them."
Hadith Referenceمسند الإمام الشافعي / كتاب البيوع / 1384
Hadith Takhrijاخرجه البخاري، البيوع، باب بيع الفضة بالفضة (2177) - ومسلم ، البيوع، باب الربا (1584)۔