Shakyamuni, the man who became known as the Buddha lived over 2,500 years ago. His words were recorded in the Pali Canon a few hundred years after his death. The Ānāpānasati Sutta , or “Breath-Mindfulness Discourse” details his instructions for using the breath (anapana) as a focus for meditation.
“And how, monks, is mindfulness of breathing developed and cultivated, so that it is of great fruit and great benefit?
“Here a monk, gone to the forest or to the root of a tree or to an empty hut, sits down; having folded his legs crosswise, set his body erect, and establishes mindfulness in front of him, ever mindful he breathes in, mindful he breathes out.
“Breathing in long, he understands: ‘I breathe in long’; or breathing out long, he understands: ‘I breathe out long’: Breathing in short, he understands: ‘I breathe in short’; breathing out short, he understands: ‘I breathe out short’.
He trains thus: ‘I shall breathe in experiencing the whole body’; He trains thus: ‘I shall breathe out experiencing the whole body. He trains thus: ‘I shall breathe in tranquilizing the bodily formation’: he trains thus: ‘I shall breathe out tranquilizing the bodily formation’.”
–from The Bare-Bones Instructions to “Mindfulness of Breathing”
by Ven. Bhante Vimalaramsi
Dhamma Sukha Meditation Center