Translated from Taishō Tripiṭaka volume 2, number 99
Thus have I heard. At one time, the Buddha was dwelling in Śrāvastī, in the Jeta Grove, in Anāthapiṇḍada’s Park.
At that time, the Bhagavān said to the bhikṣus, “You should observe the impermanence of  form. One who observes thusly, observes correctly. One who correctly observes gives rise to disenchantment. One who is disenchanted exhausts delight and craving. One who exhausts delight and craving is called liberated in mind.
“It is also such as this for observing the impermanence of  sensation,  conception,  formation, and  discrimination. One who observes thusly, observes correctly. One who correctly observes gives rise to disenchantment. One who is disenchanted exhausts craving. One who exhausts craving is called liberated in mind.
“Thusly, bhikṣus, one who is liberated in mind, if he wishes to realize, is able to realize, ‘My births have been ended. Brahmacarya has been established. What was to be done has been done, and there is self-knowledge of no further existence.’ As with observing  impermanence, it is also such as this for  suffering,  emptiness, and  not-self.”
Then the bhikṣus heard what the Buddha had said, and joyfully practiced in accordance.