Sunan At-Tirmidhi

Narrated Anas: ""When the Prophet (ﷺ) ate, he would lick his three fingers, and he said: 'If one of you drops a piece (of food) then let him remove any harm (dirt) from it and eat it, and do not leave it for Ash-Shaitan.' And he would order us to finish (clean) the dish. And he said: 'Indeed you do not know in which part of your food is the blessing.' [Abu 'Eisa said:] This Hadith is Hasan Gharib Sahih