Sahih Muslim Book 23, Hadith Number 5050.

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.

Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: When any one of you cats food he should lick his fingers, for hen does not know in what part of the food sticking to his fingers the blessing lies. This hadith has been reported on the authority of Hammad with the same chain of transmitters, but with a slight variation of wording.