SN47.38 — Pariññātasutta

There are these four foundations of mindfulness.

What four?

Here a disciple dwells contemplating the body in the body, ardent, fully aware, mindful, having put away covetousness and grief for the world.

For one dwelling contemplating the body in the body, the body becomes fully understood.

Because of the body being fully understood, the deathless is realized.

He dwells contemplating feelings in feelings, ardent, fully aware, mindful, having put away covetousness and grief for the world.

For one dwelling contemplating feelings in feelings, feelings become fully understood.

Because of feelings being fully understood, the deathless is realized.

He dwells contemplating mind in mind, ardent, fully aware, mindful, having put away covetousness and grief for the world.

For one dwelling contemplating mind in mind, the mind becomes fully understood.

Because of the mind being fully understood, the deathless is realized.

He dwells contemplating phenomena in phenomena, ardent, fully aware, mindful, having put away covetousness and grief for the world.

For one dwelling contemplating phenomena in phenomena, phenomena become fully understood.

Because of phenomena being fully understood, the deathless is realized.