Monks’ rules and their analysis
Monks’ Training (Sekhiya) 61
‘I will not teach dhamma to (someone) wearing shoes (and) who is not ill,’ is a training to be observed.”
Dhamma should not be taught to (someone) wearing shoes (and) who is not ill. Whoever out of disrespect teaches dhamma to (someone) mounted on (shoes) or (with shoes) fastened on or (with shoes) unfastened (and) who is not ill, there is an offence of wrong-doing.
There is no offence if it is unintentional … if he if the first wrong-doer.