SN54.6 — Ariṭṭhasutta
Originating in Sāvatthi.
There the Blessed One said, Develop mindfulness of breathing, disciples.
When this was said, Venerable Ariṭṭha said to the Blessed One, I develop mindfulness of breathing, venerable sir.
But how do you develop mindfulness of breathing, Ariṭṭha? the Blessed One asked.
In the past, venerable sir, sensual desire for sensual pleasures has been abandoned by me, and in the future, sensual desire for sensual pleasures will be gone. Internally and externally, perceptions of aversion have been well subdued. Thus, I breathe in mindfully, and I breathe out mindfully. This is how I develop mindfulness of breathing, venerable sir.
That is indeed mindfulness of breathing, Ariṭṭha, but it is not complete. Listen, Ariṭṭha, and I will explain how mindfulness of breathing is fully developed.
Yes, venerable sir, Venerable Ariṭṭha replied.
The Blessed One said, And how, Ariṭṭha, is mindfulness of breathing fully developed? Here, Ariṭṭha, a disciple goes to the forest, to the root of a tree, or to an empty hut, sits down, crosses his legs, keeps his body upright, and establishes mindfulness on itself. Mindfully he breathes in, and mindfully he breathes out.
Breathing in long, he knows, I breathe in long; breathing out long, he knows, I breathe out long... He trains thus: I will breathe in contemplating relinquishment; he trains thus: I will breathe out contemplating relinquishment.
This is how, Ariṭṭha, mindfulness of breathing is fully developed.