Theravāda Vinayapiṭaka
Monks’ rules and their analysis
Monks’ Training (Sekhiya) 63
… in Anāthapiṇḍika’s monastery. Now at that time the group of six monks taught dhamma to (someone) in a vehicle … “…
‘I will not teach dhamma to (someone) in a vehicle (and) who is not ill,’ is a training to be observed.”
Vehicle means: a cart, a carriage, a waggon, a chariot, a palanquin, a sedan-chair.
Dhamma should not be taught to (someone) in a vehicle (and) who is not ill. Whoever out of disrespect teaches dhamma to (someone) in a vehicle (and) who is not ill, there is an offence of wrong-doing.
There is no offence if it is unintentional … if he is the first wrong-doer.