Musnad Ahmad 1012:18

Isma'il ibn Ibrahim narrated to us, from Khalid, from Abu Qilabah, from Abu al-Ash'ath, who said: There were people selling silver from the spoils of war for [future] payment. Ubadah ibn al-Samit said: "The Messenger of Allah (ﷺ) forbade gold for gold, silver for silver, dates for dates, wheat for wheat, barley for barley, and salt for salt, except [if they are] equal for equal, like for like. So whoever increases or seeks an increase, he has committed usury (riba)."