Musnad Ahmad 298:16

Hassan ibn Musa narrated to us, he said: Hammad—meaning Ibn Zayd—narrated to us, from Ayyub, from Abu Qilabah, who said: Hisham ibn Amir arrived in Basra and found them trading gold in their stipends. He stood up and said: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) forbade the selling of gold for silver on credit, and he informed us—or he said—that that is usury."