Sahih Muslim 2035
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
وَحَدَّثَنِيهِ أَبُو بَكْرِ بْنُ نَافِعٍ ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ، - يَعْنِي ابْنَ مَهْدِيٍّ - قَالاَ حَدَّثَنَا حَمَّادٌ ، بِهَذَا الإِسْنَادِ غَيْرَ أَنَّهُ قَالَ " وَلْيَسْلُتْ أَحْدُكُمُ الصَّحْفَةَ " . وَقَالَ " فِي أَىِّ طَعَامِكُمُ الْبَرَكَةُ أَوْ يُبَارَكُ لَكُمْ " .
This hadith has been reported on the authority of Hammad with the same chain of transmitters, but with a slight variation of wording.