Sahih Muslim Book 23, Hadith Number 5046.

Chapter : The merit of licking the fingers after taking food and wiping the dish (with fingers) and eating of the fallen mouthful after removing the dirt sticking to it.

Jabir reported: I heard Allah’s Apostle (may peace be upon him) as saying: The Satan is present with any one of you in everything he does; he is present even when he eats food; so if any one of you drops a mouthful he should remove away anything filthy on it and eat it and not leave for the devil; and when he finishes (food) he should lick his fingers, for he does not know in what portion of his food the blessing lies.