Niganthasutta 4.4

4.4

Imā kho, abhaya, tisso nijjarā visuddhiyo tena bhagavatā jānatā passatā arahatā sammāsambuddhena sammadakkhātā sattānaṃ visuddhiyā sokaparidevānaṃ samatikkamāya dukkhadomanassānaṃ atthaṅgamāya ñāyassa adhigamāya nibbānassa sacchikiriyāyā”ti.

These are the three kinds of purification by wearing away that have been rightly explained by the Buddha … in order to realize extinguishment.”