وعن كعب بن مالك رضى الله عنه
قال: رأيت رسول الله صلى الله عليه وسلم يأكل بثلاث أصابع، فإذا فرغ لعقها
I saw Messenger of Allah (sallallahu alayhi wa sallam) eating with three fingers (i.e., the thumb, the index finger and the middle finger) and licking them after having finished the food.
Riyad as-Salihin 748, Book 3, The Book About the Etiquette of Eating, Hadith 22. 1 min read