Musnad Ahmad 298:2

Isma'il narrated to us, he said: Ayyub narrated to us, from Abu Qilabah, he said: The people used to buy gold with silver on credit until the time of the stipend (al-'ata'). Hisham ibn 'Amir came upon them and forbade them, and he said: "Indeed, the Messenger of Allah (peace be upon him) forbade us from selling gold for silver on credit." And he informed us—or he said: And he told us—that that is usury (riba).