Anas (May Allah be pleased with him) reported:
Whenever Messenger of Allah (ﷺ) ate food, he would lick his three fingers and say, "If anyone of you drops a morsel of food, he should remove any dirt that may have stuck on it and then eat it, and should not leave it for Satan." He (ﷺ) also commanded us that we should glean the pot, saying, "You do not know in which part of your food the blessings lies."[Muslim].
App reference: Book 3, Hadith 26
Arabic/English book reference: Book 3, Hadith 753