“And how is mindfulness of breathing fulfilled in detail? It’s when a mendicant—gone to a wilderness, or to the root of a tree, or to an empty hut—sits down cross-legged, sets their body straight, and brings mindfulness to the present. Just mindful, they breathe in. Mindful, they breathe out. Breathing in heavily they know: ‘I’m breathing in heavily.’ Breathing out heavily they know: ‘I’m breathing out heavily.’ … They practice like this: ‘I’ll breathe in observing letting go.’ They practice like this: ‘I’ll breathe out observing letting go.’ This is how mindfulness of breathing is fulfilled in detail.”

Linked Discourses 54.7 1. One Thing About Mahākappina

At Sāvatthī. Now at that time Venerable Mahākappina was sitting not far from the Buddha, cross-legged, his body set straight, and mindfulness brought to the present. The Buddha saw him, and addressed the mendicants:

“Mendicants, do you see any perturbation or trembling in that mendicant’s body?” “Sir, whenever we see that mendicant meditating—whether in the middle of the Saṅgha or alone in private—we never see any perturbation or trembling in his body.”