Sahih Muslim Book 23, Hadith Number 5049.

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.

Anas reported that when Allah’s Messenger (may peace be upon him) ate food he licked his three fingers, and he said: When any one of you drops a mouthful he should remove anything filthy from it and then eat it, and should not leave it for the Satan. He also commanded us that we should wipe the dish saying: You do not know in what portion of your food the blessing lies.