Musnad Ahmad 27:788

Waki’ narrated to us, Isra’il narrated to us, from Simak ibn Harb, from Sa’id ibn Jubayr, from Ibn ‘Umar, who said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "If you buy gold with silver, or one of them with the other, then do not let him depart from you while there is between you and him a delay."