Paṭhamabodhi sutta: Upon Awakening (1st)

So I have heard. At one time, when he was first awakened, the Buddha was staying in Uruvelā at the root of the tree of awakening on the bank of the Nerañjarā River. There the Buddha sat cross-legged for seven days without moving, experiencing the bliss of freedom. When seven days had passed, the Buddha emerged from that state of immersion. In the first part of the night, he carefully applied the mind to dependent origination, going with the grain:

“So, when this exists, this comes to be; due to the arising of this, this arises. That is: Ignorance is a requirement for choices. Choices are a requirement for consciousness. Consciousness is a requirement for name and form. Name and form are requirements for the six sense fields. The six sense fields are requirements for contact. Contact is a requirement for feeling. Feeling is a requirement for craving. Craving is a requirement for grasping. Grasping is a requirement for continued existence. Continued existence is a requirement for rebirth. Rebirth is a requirement for old age and death, sorrow, lamentation, pain, sadness, and distress to come to be. That is how this entire mass of suffering originates.”

Then, understanding this matter, on that occasion the Buddha expressed this heartfelt sentiment:

“When things become clear
to the keen, meditating brahmin,
his doubts are dispelled,
since he understands each thing and its cause.”