The Buddha explains that mindfulness while breathing in and out yields great fruit when integrated with the seven factors of awakening.
“Ānāpānassati, bhikkhave, bhāvitā bahulīkatā mahapphalā hoti mahānisaṁsā. Kathaṁ bhāvitā ca, bhikkhave, ānāpānassati kathaṁ bahulīkatā mahapphalā hoti mahānisaṁsā? Idha, bhikkhave, bhikkhu ānāpānassatisahagataṁ satisambojjhaṅgaṁ bhāveti vivekanissitaṁ virāganissitaṁ nirodhanissitaṁ vossaggapariṇāmiṁ, ānāpānassatisahagataṁ dhammavicayasambojjhaṅgaṁ bhāveti …pe… ānāpānassatisahagataṁ upekkhāsambojjhaṅgaṁ bhāveti vivekanissitaṁ virāganissitaṁ nirodhanissitaṁ vossaggapariṇāmiṁ. Evaṁ bhāvitā kho, bhikkhave, ānāpānassati evaṁ bahulīkatā mahapphalā hoti mahānisaṁsā”ti.
“|Mindfulness while breathing in and out::mindfulness during inhalation and exhalation [ānāpānassati]||, bhikkhus,::::| when cultivated and frequently practiced, is of great fruit and great benefit. And how|, bhikkhus,::::| is mindfulness while breathing in and out cultivated and frequently practiced so that it is of great fruit and great benefit? Here, |bhikkhus,::::| a bhikkhu cultivates the |awakening factor of mindfulness::the quality of mindfulness leading to full awakening, remembering as a factor of enlightenment; first of the seven awakening factors [satisambojjhaṅga]| accompanied by mindfulness while breathing in and out, which is |supported by seclusion::dependent on detachment, by means of disengagement [vivekanissita]|, |supported by dispassion::based on fading of desire [virāganissita]|, |based on ending::supported by cessation [nirodhanissita]|, and |culminates in complete relinquishment::ripens in release, culminating in letting go [vossaggapariṇāmī]|; he cultivates the |awakening factor of investigation of states::the quality of discriminative investigation into states, mental qualities and other phenomena; analyzing the teaching as a factor of enlightenment; second of the seven awakening factors [dhammavicayasambojjhaṅga]| accompanied by mindfulness while breathing in and out … he cultivates the |awakening factor of equanimity::the quality of balanced observation, non-reactivity, and mental equipoise as a factor of enlightenment; seventh of the seven awakening factors [upekkhāsambojjhaṅga]| accompanied by mindfulness while breathing in and out, which is supported by seclusion, supported by dispassion, based on ending, and culminates in complete relinquishment. Mindfulness while breathing in and out, |bhikkhus,::::| when cultivated and frequently practiced in this way, is of great fruit and great benefit.”
“|Mindfulness while breathing in and out::mindfulness during inhalation and exhalation [ānāpānassati]||, bhikkhus,::::| when cultivated and frequently practiced, is of great fruit and great benefit. And how|, bhikkhus,::::| is mindfulness while breathing in and out cultivated and frequently practiced so that it is of great fruit and great benefit? Here, |bhikkhus,::::| a bhikkhu cultivates the |awakening factor of mindfulness::the quality of mindfulness leading to full awakening, remembering as a factor of enlightenment; first of the seven awakening factors [satisambojjhaṅga]| accompanied by mindfulness while breathing in and out, which is |supported by seclusion::dependent on detachment, by means of disengagement [vivekanissita]|, |supported by dispassion::based on fading of desire [virāganissita]|, |based on ending::supported by cessation [nirodhanissita]|, and |culminates in complete relinquishment::ripens in release, culminating in letting go [vossaggapariṇāmī]|; he cultivates the |awakening factor of investigation of states::the quality of discriminative investigation into states, mental qualities and other phenomena; analyzing the teaching as a factor of enlightenment; second of the seven awakening factors [dhammavicayasambojjhaṅga]| accompanied by mindfulness while breathing in and out … he cultivates the |awakening factor of equanimity::the quality of balanced observation, non-reactivity, and mental equipoise as a factor of enlightenment; seventh of the seven awakening factors [upekkhāsambojjhaṅga]| accompanied by mindfulness while breathing in and out, which is supported by seclusion, supported by dispassion, based on ending, and culminates in complete relinquishment. Mindfulness while breathing in and out, |bhikkhus,::::| when cultivated and frequently practiced in this way, is of great fruit and great benefit.”
“Ānāpānassati, bhikkhave, bhāvitā bahulīkatā mahapphalā hoti mahānisaṁsā. Kathaṁ bhāvitā ca, bhikkhave, ānāpānassati kathaṁ bahulīkatā mahapphalā hoti mahānisaṁsā? Idha, bhikkhave, bhikkhu ānāpānassatisahagataṁ satisambojjhaṅgaṁ bhāveti vivekanissitaṁ virāganissitaṁ nirodhanissitaṁ vossaggapariṇāmiṁ, ānāpānassatisahagataṁ dhammavicayasambojjhaṅgaṁ bhāveti …pe… ānāpānassatisahagataṁ upekkhāsambojjhaṅgaṁ bhāveti vivekanissitaṁ virāganissitaṁ nirodhanissitaṁ vossaggapariṇāmiṁ. Evaṁ bhāvitā kho, bhikkhave, ānāpānassati evaṁ bahulīkatā mahapphalā hoti mahānisaṁsā”ti.