Saṃyuktāgama 219. [First Discourse on the Path to Nirvāṇa]
Thus have I heard. At one time the Buddha was staying at Sāvatthī in Jeta’s Grove, Anāthapiṇḍika’s Park.
At that time the Blessed One said to the monks: “I will now teach you the path to Nirvāṇa. What is the path to Nirvāṇa? That is, contemplate the eye as impermanent. Forms, eye-consciousness, and feeling arisen in dependence on eye-contact that is experienced within, be it painful, pleasant, or neutral, that is also impermanent.
“The ear … the nose … the tongue … the body … the mind is also like that. This is called the path to Nirvāṇa.”
When the Buddha had spoken this discourse, hearing what the Buddha had said the monks were delighted and received it respectfully.