Jabir (Radhiallahu anhu) reported:
Messenger of Allah (Sallallaahu ‘alaihi wasallam) commanded the licking of fingers and the gleaning of the dish, saying, “You do not know in which portion the blessing lies.”
This Hadith underlines the wisdom behind licking the fingers and the vessel. The eater never knows whether a blessing underlies what has been eaten by him, or the particles sticking to his fingers and vessels contain it. Those particles, therefore, should not be wasted; they should be made part of food by being licked. They may be invested with a blessing and proved more useful for the eater’s health and strength. Moreover, by this way he escapes disregarding the Bounty of Allah.
Riyad-us-Saliheen – Hadith No:750
Please Share this Hadith with your relatives & friends and may Allah SWT reward you.