Saṃyukta Āgama Sutta 287 (1)

Thus have I heard. At one time, the Buddha was staying at the Jeta Grove in Anathapindika's park in Savatthi. At that time, the Blessed One addressed the bhikkhus: "I recall, when I had not yet attained perfect enlightenment, alone in a secluded place, diligently engaged in meditation, I thought thus: 'What is the cause that aging and death exist? Due to what condition does aging and death arise?' Then, with right reflection, arose in me the uninterrupted knowledge of reality: 'With birth as condition, aging and death exist; due to the condition of birth, aging and death arise.' Thus, with clinging, craving, feeling, contact, the six sense bases, name-and-form."

The Blessed One told the bhikkhus: "I recall, in a past life when I had not yet attained perfect enlightenment, alone in a secluded place, single-mindedly concentrated on contemplation. This thought arose in my mind: 'What is the cause that aging and death come into being? Due to what condition does aging and death arise?' Then I entered into right reflection. After right reflection, the uninterrupted knowledge of reality arose, knowing that with birth, aging and death follow, due to the condition of birth, aging and death arise. Thus, step by step, tracing back, I realized that the arising of living beings is due to the appearance of the three realms and the world of vessels, thus living beings come into being; with the three realms as condition, living beings are born. The appearance of the three realms and the world of vessels is due to sentient beings clinging to the five aggregates of worldly phenomena, thus existence comes into being; with the clinging of sentient beings as condition, existence is born. Sentient beings have craving for the five aggregates, thus clinging arises; due to craving, the phenomenon of clinging arises. Sentient beings have feelings towards the five aggregates, thus craving arises; due to feelings, craving is born. Sentient beings have contact between the six sense faculties and the six sense objects, thus feelings arise; due to contact, feelings are born. Sentient beings have the six external sense bases and the six internal sense bases, thus contact arises; due to the six internal and external sense bases, contact is produced. Sentient beings have name-and-form, thus the six sense bases come into being; with name-and-form as condition, the six sense bases are produced."