Musnad Abu Ya'la al-Mawsili 117:103

Ubayd Allah al-Qawariri narrated to us, Abd al-Rahman ibn Mahdi narrated to us, Hammad ibn Salamah narrated to us, from Thabit, from Anas, that the Prophet (peace and blessings of Allah be upon him) used to lick his three fingers when he ate. And he said: "If the morsel of one of you falls, let him remove whatever filth is upon it, and let him eat it, and not leave it for the Shaytan." And he said: "And let him wipe the dish clean, for you do not know in which part of your food the blessing lies."