Musnad Ahmad 27:1426

Husayn ibn Muhammad narrated to us, Khalaf—meaning Ibn Khalifah—narrated to us, from Abu Janab, from his father, from Ibn Umar, who said: The Messenger of Allah (ﷺ) said: "Do not sell a dinar for two dinars, nor a dirham for two dirhams, nor a sa' for two sa's, for I fear for you *ar-rima'*." And *ar-rima'* is usury (*riba*). A man stood up to him and said: "O Messenger of Allah, what about a man who sells a horse for horses, or a she-camel for camels?" He said: "There is no harm in it if it is hand-to-hand."