Here a Monk, gone to the forest or to the foot of a tree, or to an empty hut, sits down; having folded his legs crosswise, set his body erect, and established mindfulness in front of him, ever mindful he breathes in, mindful he breathes out:

Breathing in long, he understands: I breathe in long; or breathing out long,

he understands: I breathe out long.

Breathing in short, he understands: I breathe in short; or breathing out short, he understands: I breathe out short.

He trains thus: I shall breathe in experiencing the whole body;

he trains thus: I shall breathe out experiencing the whole body.

He trains thus: I shall breathe in calming the bodily sensations;

he trains thus: I shall breathe out calming the bodily sensations.

As he resides thus diligent, dedicated, and determined, his memories and thoughts based on the household life are abandoned; with their abandoning his mind becomes steadied internally, quieted, brought to singleness, and concentrated.

That is how a Monk develops mindfulness of the body.

(Reference: MN 119)