[874] Origin at Sāvatthī. Now at that time the group-of-six bhikkhus, while walking behind, taught the Dhamma to one walking in front. …
"I shall not, while walking behind, teach the Dhamma to one who is not sick who is walking in front" — thus the training is to be undertaken.
The Dhamma is not to be taught by one walking behind to one who is not sick who is walking in front. Whatever bhikkhu, out of disrespect, while walking behind teaches the Dhamma to one who is not sick who is walking in front, incurs an offense of wrong-doing (dukkaṭa).
There is no offense: if it is unintentional; if he is unmindful; if he does not know; if the person is sick; in emergencies; if he is insane; if he is the first offender.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.