Musnad Abu Ya'la al-Mawsili 117:38

Ibrahim ibn al-Hajjaj narrated to us, Hammad narrated to us, from Thabit, from Anas, that the Prophet (may Allah bless him and grant him peace) used to lick his three fingers when he ate food. He said: "If a morsel of food falls from one of you, let him remove the filth from it and eat it, and not leave it for the Shaytan." He commanded us to wipe the dish clean, and he said: "Indeed, one of you does not know in which part of his food the blessing lies."