Abd al-Rahman bin Abi Bakrah narrates from his father (Sayyiduna Abu Bakrah, may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) forbade selling silver for silver and gold for gold except equal for equal, and ordered us to buy silver with gold and gold with silver however we wished. A man asked him (Sayyiduna Abu Bakrah): "Hand to hand?" He said: This is how I heard it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10502