From Sahih Muslim (Hadith 4073):
Abd al-Rabman b. Abia Bakra reported on the authority of his father that Allah's Messenger (ﷺ) forbade the sale of gold for gold, and silver for silver except equal for equal, and commanded us to buy silver for gold as we desired and buy gold for silver as we desired. A person asked him (about the nature of payment), whereupon he said:It is to be made on the spot. This is what I heard (from Allah's Messenger (may peace be upon him)
Reference: Muslim 4073