54 Ānāpāna Saɱyutta
1. Eka-Dhamma Vaggo
Connected Discourses on Breathing
Book 1: One Thing
Translated from the Pali by Bhikkhu Bodhi
"Bhikkhus, do you develop mindfulness of breathing?"
When this was said, the Venerable Ariṭṭha said to the Blessed One: "Venerable sir, I develop mindfulness of breathing." 
"But in what way, Ariṭṭha, do you develop mindfulness of breathing?"
"I have abandoned sensual desire for past sensual pleasures, venerable sir, I have gotten rid of sensual desire for future sensual pleasures, and I have thoroughly dispelled perceptions of aversion towards things internally and externally. Just mindful I breathe in, mindful I breathe out. It is in this way, venerable sir, that I develop mindfulness of breathing."
"That is mindfulness of breathing, Ariṭṭha, I do not say that it is not. But as to how mindfulness of breathing is fulfilled in detail, Ariṭṭha, listen and attend closely, I will speak."
"Yes, venerable sir," the Venerable Ariṭṭha replied. The Blessed One said this:
"And how, Ariṭṭha, is mindfulness of breathing fulfilled in detail? Here, Ariṭṭha, a bhikkhu, having gone to the forest, to the foot of a tree, or to an empty hut, sits down. Having folded his legs crosswise, straightened his body, and set up mindfulness in front of him, just mindful he breathes in, mindful he breathes out.... He trains thus: 'Contemplating relinquishment, I will breathe in'; he trains thus: 'Contemplating relinquishment, I will breathe out.'
"It is in this way, Ariṭṭha, that mindfulness of breathing is fulfilled in detail."
Spk: Ariṭṭha had explained his own (attainment of) the nonreturner's path [Spk-pt: because he spoke obliquely of the eradication of the five lower fetters], but the Buddha explained the insight practice to gain the path of arahantship.