Saṃyukta Āgama 34
Thus have I heard. At one time the Bhagavān was dwelling in Vārāṇasī, at the Deer Park of Ṛṣipatana. At that time, the Bhagavān told a group of five bhikṣus, “Form does not exist as a self. If form existed as a self, then form would not be associated with the arising of illness and suffering. Regarding form, it is also not possible to cause it to be like this, or not like this, because form is not oneself. From form and the arising of illness and suffering, one also grasps the desire to make form like this, or not like this. For sensation, conception, synthesis, and discrimination, it is also such as this.
“Bhikṣus, tell me what you think: is form permanent or impermanent?” The bhikṣus addressed the Buddha, saying, “Impermanent, Bhagavān.” “Bhikṣus, is that which is impermanent, suffering?” The bhikṣus addressed the Buddha, saying, “It is suffering, Bhagavān.” “Bhikṣus, regarding these impermanent and afflicting dharmas, easily subject to change, does a well-learned venerable disciple perceive in these a self or a non-self, and thereby dwell in appearances?” The bhikṣus addressed the Buddha saying, “No, Bhagavān, and for sensation, conception, synthesis, and discrimination, it is also such as this.” “For this reason, bhikṣus, all form that exists—whether in the past, the future, or the present; internal or external; coarse or fine; appealing or unappealing; far or near—all these are not a self, not a true self. Correct contemplation of sensation, conception, synthesis, and discrimination is also such as this.
“Bhikṣus, regarding the Five Skandhas, a well-learned venerable disciple perceives they are not a self, not a true self, and contemplates thusly. Regarding the various realms, because there is no clinging, there is no suffering, and because there is no suffering, there is self-awakening and Nirvāṇa. ‘My births have come to an end, Brahmacarya has been established, what was to be done has been done, and there is the self-realization of no further suffering.’”
After the Buddha had spoken this sūtra, the group of five bhikṣus did not give rise to outflows, and their minds attained liberation. After the Buddha had spoken this sūtra, then the bhikṣus heard what the Buddha had said, and blissfully practiced in accordance.