Musnad Ahmad 30:451

Abu Kamil narrated to us, Zuhayr narrated to us, Suhayl narrated to us, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever sleeps while there is *ghamr* (grease or residue of food) on his hand and he does not wash it, then if something befalls him, let him blame none but himself."