Majjhima Nikaya 3:60
SUJATODīghaṁ vā assasanto ‘dīghaṁ assasāmī’ti pajānāti, dīghaṁ vā passasanto ‘dīghaṁ passasāmī’ti pajānāti;
Breathing in heavily they know: ‘I’m breathing in heavily.’ Breathing out heavily they know: ‘I’m breathing out heavily.’