Theravāda Vinayapiṭaka
Monks’ rules and their analysis
Monks’ Training (Sekhiya) 42
… in Anāthapiṇḍika’s monastery. Now at that time the group of six monks, while eating, put the whole hand into the mouth … “…
‘I will not put the whole hand into the mouth while eating,’ is a training to be observed.”
One should not put the whole hand into the mouth while eating. Whoever out of disrespect puts the whole hand into the mouth while eating, there is an offence of wrong-doing.
There is no offence if it is unintentional … if he is the first wrong-doer.