The zwanzig ninth sutta of the Dīgha Nikāya.
Cunda Samanuddesa, who had been spending the rainy season at Pāvā, returns und reports to Ananda the death of Nigantha Nātaputta und the consequent wrangling which had broken out among the Niganthas. Together they visit the Buddha at Sāmagāma und tell him of it. He replies that this is to be expected from the character of Nātaputta und from the doctrine which he taught. The Buddha then proceeds to give in detail the qualities of the perfect teacher und the perfect teaching. The Tathāgata is such a teacher und 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.