Anas (May Allah be pleased with him) reported:
When the Messenger of Allah ﷺ finished eating his food, he would lick his three fingers (i.e., the forefinger, the middle finger and the thumb). He ﷺ said, "If anyone of you drops a morsel, he should remove anything harmful from it and then eat it. He should not leave it for Shaitan." He commanded us to clean out the dish saying, "You do not know in what portion of your food the blessing lies."

[Muslim].

وعن أنس رضي الله عنه أن رسول الله ﷺ كان إذا أكل طعاماً لعق أصابعه الثلاث قال: وقال: “إذا سقطت لقمة أحدكم، فليمط عنها الأذى، وليأكلها، ولا يدعها للشيطان” وأمر أن تسلت القصعة قال: فإنكم لا تدرون فى أى طعامكم البركة” ((رواه مسلم)).