Musnad Abu Ya'la al-Mawsili 155:194

Abu Hammam narrated to us, Yahya ibn Yaman narrated to us, from Hisham, from Ibn Sirin, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade *al-sarf* (the exchange of currency), a dirham for two dirhams. And Abu Bakr, and Umar, and Uthman [also forbade it].