كتـــــاب أدب الطعام
3
The Book About the Etiquette of Eating
Ka'b bin Malik (May Allah be pleased with him) reported:
I saw Messenger of Allah (ﷺ) eating with three fingers (i.e., the thumb, the index finger and the middle finger) and licking them after having finished the food.

[Muslim].

وعن كعب بن مالك رضى الله عنه قال‏:‏ رأيت رسول الله صلى الله عليه وسلم يأكل بثلاث أصابع ، فإذا فرغ لعقها‏.‏ ‏
Sunnah.com reference : Book 3, Hadith 22
Arabic/English book reference : Book 3, Hadith 749