Majjhima Nikaya 2:19
SUJATOrassaṁ vā assasanto ‘rassaṁ assasāmī’ti pajānāti, rassaṁ vā passasanto ‘rassaṁ passasāmī’ti pajānāti.
When breathing in lightly they know: ‘I’m breathing in lightly.’ Breathing out lightly they know: ‘I’m breathing out lightly.’