Musnad Abu Ya'la al-Mawsili 155:355

Abu Sa'id al-Ashajj narrated to us, Ziyad ibn al-Hasan ibn al-Furat al-Qazzaz narrated to us, from his father, from his grandfather, from Abu Hazim, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "There is no tree in Paradise except that its trunk is made of gold."