The twenty fourth sutta of the Majjhima Nikāya. Sāriputta visits Punna Mantāniputta in Andhavana and asks him why he lives the higher life. To attain absolute Nibbāna, answers Punna, and, on being questioned further as to the nature of Nibbāna, he explains that Nibbāna is the goal and can only be attained by passing through various successive stages purity of life, purity of heart, purity of view, purity through dispelling doubts, purity through full insight into paths, right and wrong, into the path to be followed, and the purity which arises from insight. It may be compared to a journey of Pasenadi from Sāvatthi to Sāketa, by means of relays of seven carriages.
It is said in the introduction to the sutta that Sāriputta had been awaiting the opportunity of a discussion with Punna ever since he heard the monks at Veluvana in Rājagaha speak of him to the Buddha in terms of the highest praise. But this opportunity did not arise until later, when Punna visited Sāvatthi. Punna was unaware of the identity of Sāriputta until the end of his discourse (M.i.145 51).
The Mahāvamsa Tīkā mentions (MT.552f ) that once Ambapāsānavāsī Cittagutta preached this sutta to a very large assembly of monks and nuns at the Lohapāsāda in Anurādhapura, and in his exposition of the sutta included a short account of the relies enshrined in the Mahā Thūpa.
It has been suggested by Dr. Neumann that the upatisapasina mentioned in Asoka's Bhabru Edict, refers to this sutta. But see Rhys Davids, J.R.A.S.1893, and Mukherji, Asoka, 118f., n.8.