The twenty ninth sutta of the Dīgha Nikāya.
Cunda Samanuddesa, who had been spending the rainy season at Pāvā, returns and reports to Ananda the death of Nigantha Nātaputta and the consequent wrangling which had broken out among the Niganthas. Together they visit the Buddha at Sāmagāma and tell him of it. He replies that this is to be expected from the character of Nātaputta and from the doctrine which he taught. The Buddha then proceeds to give in detail the qualities of the perfect teacher and the perfect teaching. The Tathāgata is such a teacher and the Dhamma such a teaching (D.iii.117 41).
In the Pāsādika Sutta the threefold training is called brahmacariyā. DA.i.179; MA.i.275.