Musnad Ahmad 27:1323

Husayn ibn Muhammad narrated to us, he said: Isra'il narrated to us, from Simak, from Sa'id ibn Jubayr, from Ibn 'Umar, who said: I asked the Prophet, peace and blessings of Allah be upon him: "Should I buy gold with silver, and silver with gold?" He said: "If you take one of them for the other, then do not let your companion depart from you while there is between you and him a confusion [i.e., an unsettled transaction]."