Sahih Muslim 2031
Sahih Muslim · Book 36: The Book of Drinks · It is recommended to lick one's fingers and wipe the bowl, and to eat a piece of food that is dropped after removing any dirt on it. It is disliked to wipe one's hand before licking it, because of the possibility that the blessing of the food may be in that remaining part. The Sunnah is to eat with three fingers
Ibn 'Abbas reported Allah's Messenger (ﷺ) as saying:
حَدَّثَنِي هَارُونُ بْنُ عَبْدِ اللَّهِ ، حَدَّثَنَا حَجَّاجُ بْنُ مُحَمَّدٍ ، ح وَحَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ ، أَخْبَرَنِي أَبُو عَاصِمٍ ، جَمِيعًا عَنِ ابْنِ جُرَيْجٍ ، ح وَحَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ ، - وَاللَّفْظُ لَهُ - حَدَّثَنَا رَوْحُ بْنُ عُبَادَةَ ، حَدَّثَنَا ابْنُ جُرَيْجٍ ، قَالَ سَمِعْتُ عَطَاءً ، يَقُولُ سَمِعْتُ ابْنَ عَبَّاسٍ ، يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا أَكَلَ أَحَدُكُمْ مِنَ الطَّعَامِ فَلاَ يَمْسَحْ يَدَهُ حَتَّى يَلْعَقَهَا أَوْ يُلْعِقَهَا " .
When any one of you eats food he should not wipe his hand until he has licked it or have it licked.