SN54.3 — Suddhikasutta

Mindfulness of breathing when developed and cultivated, is of great fruit and great benefit.

And how is mindfulness of breathing developed and cultivated so that it is of great fruit and great benefit?

Here a disciple goes to the forest, to the root of a tree, or to an empty hut, sits down, crosses his legs, holds his body erect, and establishes mindfulness on itself.

Ever mindful, he breathes in; ever mindful, he breathes out...

He trains thus: I shall breathe in contemplating relinquishment; he trains thus: I shall breathe out contemplating relinquishment.

It is in this way that mindfulness of breathing, when developed and cultivated, is of great fruit and great benefit.