Saṃyutta Nikāya 22
Connected Discourses on the Aggregates
6. Seclusion
At Savatthi. “Bhikkhus, make an exertion in seclusion. A bhikkhu who is secluded understands things as they really are.
“And what does he understand as it really is? The origin and passing away of form; the origin and passing away of feeling; the origin and passing away of perception; the origin and passing away of volitional formations; the origin and passing away of consciousness.
“And what, bhikkhus, is the origin of form?…”