Mishkat al-Masabih 21 Foods كتاب الأطعمة 8

وَعَنِ ابْنِ عَبَّاسٍ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «إِذَا أَكَلَ أَحَدُكُمْ فَلَا يمسحْ يدَه حَتَّى يلعقها أَو يلعقها»

Ibn ‘Abbas reported the Prophet as saying, “When one of you eats he must not wipe his hand till he licks it, or gives it to someone to lick.” (Bukhari and Muslim.)