Sahih Muslim Book 23, Hadith Number 5037.

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 ‘Abbas reported Allah’s Messenger (may peace be upon him.) as saying: When any one of you eats food he should not wipe his hand until he has licked it himself or has given it to someone else to lick.