Abu al-Minhal (may Allah have mercy on him) said that when they were asked about exchange (sarf), both of them (may Allah be pleased with them) said that the Messenger of Allah (peace and blessings be upon him) "forbade selling silver for gold on credit."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10498