Musnad Abu Ya'la al-Mawsili 152:143

Abu Khaythama narrated to us, Waki' narrated to us, al-A'mash narrated to us, from Abu Wa'il, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Paradise is closer to one of you than the strap of his sandal." And the Fire is like that.