The twenty ninth sutta of the Dígha Nikáya.
Cunda Samanuddesa, who had been spending the rainy season at Pává, returns and reports to Ananda the death of Nigantha Nátaputta and the consequent wrangling which had broken out among the Niganthas. Together they visit the Buddha at Sámagáma and tell him of it. He replies that this is to be expected from the character of Nátaputta 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 Tathágata is such a teacher and the Dhamma such a teaching (D.iii.117 41).
In the Pásádika Sutta the threefold training is called brahmacariyá. DA.i.179; MA.i.275.