Sayyiduna Jabir (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "When a morsel falls from one of you, let him pick it up, clean it and eat it, and not leave it for Shaytan. And let him lick his hand, or have it licked, before wiping it with a napkin, for he does not know in which part of his food the blessing lies."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14617
Sayyiduna Anas bin Malik (may Allah be pleased with him) said that when the Messenger of Allah (peace and blessings be upon him) ate food, he would lick his three fingers and say: "When your morsel falls, let him clean it and eat it, and not leave it for Shaytan." And he ordered us to wipe the dish clean, and said: "None of you knows in which food the blessing lies."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14618