Riyad as-Saliheen 753
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].
وعن أنس رضى الله عنه قال: كان رسول الله ﷺ إذا أكل طعاماً، لعق أصابعه الثلاث، وقال:"إذا سقطت لقمة أحدكم فليأخذها، وليمط عنها الأذى، وليأكلها،ولا يدعها للشيطان" وأمرنا أن نسلت القصعة وقال: "إنكم لا تدرون في أي طعامكم البركة" .
Classification
Sahih (Authentic)
References
Riyad as-Saliheen 753
Riyad as-Saliheen Book of Etiquette of Eating, Hadith 26