What now is Right Attentiveness? […] - But how does the disciple dwell in the contemplation of the body? There the disciple retires to the forest, to the foot of a tree, or to a solitary place, sits himself down, with legs crossed, body erect, and with attentiveness fixed before him. With attentive mind he breathes in, with attentive mind he breathes out. When making a long inhalation, he knows: I make a long inhalation; when making a long exhalation, he knows: I make a long exhalation. When making a short inhalation, he knows: I make a short inhalation; when making a short exhalation, he knows: I make a short exhalation. Clearly perceiving the entire (breath-) body, I will breathe out: thus he trains himself. Calming this bodily function, (kaya-sankhara), I will breathe in: thus he trains himself; calming this bodily function, I will breathe out: thus he trains himself.

(Buddha Sakyamuni)