Musnad Ahmad 27:1179

Yahya ibn Adam narrated to us, Isra'il narrated to us, from Simak, from Sa'id ibn Jubayr, from Ibn Umar, who said: I asked the Messenger of Allah, peace and blessings of Allah be upon him, "May I buy gold with silver, or silver with gold?" He said: "If you buy one of them with the other, then do not let your companion depart from you while there is still a doubt between you and him."