When one of you eats, he must not wipe his hand with a handkerchief till he licks it or gives it to someone to lick.  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا أَكَلَ أَحَدُكُمْ فَلاَ يَمْسَحَنَّ يَدَهُ بِالْمِنْدِيلِ حَتَّى يَلْعَقَهَا أَوْ يُلْعِقَهَا  

The Prophet ﷺ used to eat with three fingers and not wipe his before licking it.  

أَنَّ النَّبِيَّ ﷺ كَانَ يَأْكُلُ بِثَلاَثِ أَصَابِعَ وَلاَ يَمْسَحُ يَدَهُ حَتَّى يَلْعَقَهَا