Majjhima Nikaya 2:20
SUJATO‘Sabbakāyapaṭisaṁvedī assasissāmī’ti sikkhati, ‘sabbakāyapaṭisaṁvedī passasissāmī’ti sikkhati.
They practice like this: ‘I’ll breathe in experiencing the whole body.’ They practice like this: ‘I’ll breathe out experiencing the whole body.’