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.''
[Muslim]
Commentary:
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
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