Sahih Muslim Book 23, Hadith Number 5040.

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.

Ibn Ka’b b. Malik reported on the authority of his father that the Messenger of Allah (may peace be upon him) used to eat (food) with three fingers and he licked his hand before wiping it (with towel).