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. … They practice like this: ‘I’ll breathe in observing letting go.’ They practice like this: ‘I’ll breathe out observing letting go.’ That’s how this immersion due to mindfulness of breathing is developed and cultivated so that it’s peaceful and sublime, a delectable and blissful meditation. And it disperses and settles unskillful qualities on the spot whenever they arise.”

Linked Discourses 54.10 1. One Thing With Kimbila

So I have heard. At one time the Buddha was staying near Kimbilā in the Freshwater Mangrove Wood. Then the Buddha said to Venerable Kimbila, “Kimbila, how is immersion due to mindfulness of breathing developed and cultivated so that it is very fruitful and beneficial?”

When he said this, Kimbila kept silent. For a second time … And for a third time, the Buddha said to him, “How is immersion due to mindfulness of breathing developed and cultivated so that it is very fruitful and beneficial?” And a second time and a third time Kimbila kept silent.

When he said this, Venerable Ānanda said to the Buddha, “Now is the time, Blessed One! Now is the time, Holy One! Let the Buddha speak on immersion due to mindfulness of breathing. The mendicants will listen and remember it.”