Musnad Ahmad 27:435

'Abd al-Razzaq narrated to us, Isra'il informed us, from Simak, from Sa'id ibn Jubayr, from Ibn 'Umar, that he asked the Prophet, may Allah bless him and grant him peace: "May I buy gold with silver?" He said: "If you take one of them, then do not let your companion depart from you while there is still something between you and him that is unclear."