SN 52.9 Ambapālivana Sutta: In Ambapālī’s Mango Grove
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[He] who is released through right gnosis often dwells with a mind well-established in these four establishings of mindfulness.
Sāriputta asks why Anuruddha looks so bright, and he replies that it is due to developing the four kinds of mindfulness meditation, explaining that (and demonstrating why) even Arahants continue to practice the Four Satipaṭṭhāna.