Saṃyuktāgama 222. [First Discourse on Understanding]
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: “You should understand all things that are to be understood, all things that are to be discerned. Listen and pay proper attention to what I shall teach you. What are all things that are to be understood, all things that are to be discerned?
“Monks, the eye is a thing to be understood, a thing to be discerned. Forms, eye-consciousness, eye-contact, and feeling arisen in dependence on eye-contact and experienced within, be it painful, pleasant, or neutral, all these are things to be understood, things to be discerned.
“The ear … the nose … the tongue … the body … the mind is also like that.”
When the Buddha had spoken this discourse, hearing what the Buddha had said the monks were delighted and received it respectfully.