Saṃyutta Nikāya 22
Connected Discourses on the Aggregates
40. In Accordance with the Dhamma (2)
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.”