The Book of Drinks
وَحَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ، حَدَّثَنَا بَهْزٌ، حَدَّثَنَا وُهَيْبٌ، حَدَّثَنَا سُهَيْلٌ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِذَا أَكَلَ أَحَدُكُمْ فَلْيَلْعَقْ أَصَابِعَهُ فَإِنَّهُ لاَ يَدْرِي فِي أَيَّتِهِنَّ الْبَرَكَةُ " .Abu Huraira reported Allah's Messenger (ﷺ) as saying:When any one of you eats food he should lick his fingers, for he does not know in what part of the food sticking to his fingers the blessing lies
From Sahih Muslim, whose reports the scholars accept as authentic.