[1315] And how, bhikkhus, is mindfulness of breathing (ānāpānasati) developed, how is it cultivated, so that it is of great fruit and great benefit? Here, bhikkhus, a bhikkhu, gone to the forest, or gone to the root of a tree, or gone to an empty dwelling, sits down; having folded his legs crosswise, set his body erect, and established mindfulness in front of him, mindful he breathes in, mindful he breathes out. (To be expanded in full up to: he trains thus, "Contemplating relinquishment (paṭinissagga) I shall breathe in"; he trains thus, "Contemplating relinquishment I shall breathe out.") Bhikkhus, mindfulness of breathing developed thus, cultivated thus, is of great fruit and great benefit.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.