Theravāda Vinayapiṭaka
Monks’ rules and their analysis
Monks’ Training (Sekhiya) 68
… in Anāthapiṇḍika’s monastery. Now at that time the group of six monks, having sat down on the ground, taught dhamma to (someone) sitting on a seat … “…
‘Having sat down on the ground, I will not teach dhamma to (someone) sitting on a seat (and) who is not ill,’ is a training to be observed.”
Having sat down on the ground, dhamma should not be taught to (someone) sitting on a seat (and) who is not ill. Whoever out of disrespect, having sat down on the ground, teaches dhamma to (someone) sitting on a seat (and) who is not ill, there is an offence of wrong-doing.