Saṁyutta Nikāya 54.2 1.

Bojjhaṅgasutta

Ekadhammavagga Bojjhaṅgasutta “Ānāpānassati, bhikkhave, bhāvitā bahulīkatā mahapphalā hoti mahānisaṁsā.

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

Kathaṁ bhāvitā ca, bhikkhave, ānāpānassati kathaṁ bahulīkatā mahapphalā hoti mahānisaṁsā?

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

Idha, bhikkhave, bhikkhu ānāpānassatisahagataṁ satisambojjhaṅgaṁ bhāveti vivekanissitaṁ virāganissitaṁ nirodhanissitaṁ vossaggapariṇāmiṁ, ānāpānassatisahagataṁ dhammavicayasambojjhaṅgaṁ bhāveti …pe…

Here a disciple develops the mindfulness enlightenment factor accompanied by mindfulness of breathing, which is based on seclusion, dispassion, cessation, and leads to letting go. He develops the investigation-of-states enlightenment factor accompanied by mindfulness of breathing...

ānāpānassatisahagataṁ upekkhāsambojjhaṅgaṁ bhāveti vivekanissitaṁ virāganissitaṁ nirodhanissitaṁ vossaggapariṇāmiṁ.

He develops the equanimity enlightenment factor accompanied by mindfulness of breathing, which is based on seclusion, dispassion, cessation, and leads to letting go.

Evaṁ bhāvitā kho, bhikkhave, ānāpānassati evaṁ bahulīkatā mahapphalā hoti mahānisaṁsā”ti. Dutiyaṁ.

Thus developed and cultivated mindfulness of breathing is of great fruit and great benefit.