Trường Bộ Kinh (Digha Nikaya) 1:1316
SUJATOTheratarena, ānanda, bhikkhunā navakataro bhikkhu nāmena vā gottena vā āvusovādena vā samudācaritabbo.
A more senior mendicant ought to address a more junior mendicant by name or clan, or by saying ‘reverend’.