From “Teachings on Mindfulness”

And how, monks, does a monk abide contemplating the body as body? Here a monk, having gone into the forest, or to the root of a tree, or to an empty place, sits down cross-legged, holding his body erect, having established mindfulness before him. Mindfully he breathes in, mindfully he breathes out. Breathing in a long breath, he knows that he breathes in a long breath, and breathing out a long breath, he knows that he breathes out a long breath. Breathing in a short breath, he knows that he breathes in a short breath, and breathing out a short breath, he knows that he breathes out a short breath.