At Savatthi. “Bhikkhus, when a bhikkhu is practising in accordance with the Dhamma, this is what accords with the Dhamma: he should dwell contemplating impermanence in form … as above … he is freed from suffering, I say.”
SN22.40: Dutiyaanudhammasutta
Dutiyaanudhammasutta: In Accordance with the Dhamma (2) - translated by Bhikkhu Bodhi
Translations:Thanissaro Bhikkhu
Translated by Bhikkhu Bodhi. Source text via SuttaCentral.