Musnad Ahmad 1302:45

Abu Mu'awiyah narrated to us, Hisham ibn Hassan al-Qardusi narrated to us, from Qays ibn Sa'd, from a man who narrated to him from Abu al-Darda', who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked about the wealth of the ruler. He said: "Whatever Allah brings to you from it without you asking for it, and without you coveting it, then eat it and possess it." He said: And al-Hasan said: "There is no harm in it, as long as one does not travel to it or covet it."